Wave their hands around until the music stops 
A string quartet can. A full orchestra, not so much.
There’s also the story of Oscar Levant warning a hot-tempered (and incompetent) conductor, “If you ball me out one more time, I’ll start following your tempo”.
My musician brother (double bass) told me during the last years that Eugene Ormandy conducted the Philadelphia Symphony Orchestra, they usually followed the first violinist’s tempo.
So, I guess I’m the only actual conductor on this board, huh?
To address the comment that the orchestra is the conductor’s instrument, that is very much so. When I conduct I feel as if I’m playing the players. What comes out is (hopefully) what I think the music ought to sound like, and nobody else’s interpretation.
And working with amateurs, I’d have to say that 90% of my job is keeping the ensemble together, and trust me when I say it’s a whole lot more than pretending to be a metronome.
And yes, the gestures have very definite meaning. A few years ago I was rehearsing a light opera, and one song began with a big timpani roll. The ff in the part was very clear, and the timpanist had played the part under my baton many times. There was no doubt what I wanted. So the vocal director took his turn in the driver’s seat. This was a pretty novice conductor. He started the song, and the timp roll was weak, wimpy, quiet, and just plain bad.
So I stopped the music and pulled him aside. “Stuart”, I said, “If you want Carol to play it right, you have to hit her over the head with your gesture. Hard. Make it big.” So he tried it again the way I said, and this big, loud wonderful chord filled the room. Everyone in the room broke out in big smiles.
On the comment that every performance is different – I recently got my hands on 2 DVDs of performances I did last year. Two performances, separated by a week. The running time on one was 106 minutes. On the other it was 107 minutes. I guess I’m a creature of habit.
One of the bands that I was in had a mediocre conductor. We called him “Benny Diode.”
He was a semiconductor.
Ba-boom - crash
[sub]Thank you very much, I’ll be here all week.[/sub]
(giggiling idiotically at “Benny Diode.”)
I got a crash course in the importance of conducting the year I was an assistant drum major of our community band. Both my cousin (MA in music) and my conductor had taught me the basics (“put your hands here”) but that was about it. So first time in front of my Band, I have them strike up one of our marching songs, something they’ve played since day 1 back in '78.
And I notice that the song is going slower and slower and slower…
I glanced over at the bass drummer, who is by far the most important indicator of tempo in a marching band outside of the drum major. He’s a friend of mine, and the poor guy was struggling to beat at the tempo I was giving the Band. It was obvious that (a) I was a horrible conductor and (b) that yes, the musicians were trying their damndest to follow my mediocre lead. At this point of realization I just cut them off and apologized, and did some diligent work conducting with a metronome in 120 so I could get the standard marching speed locked in my head.
I remember telling this to my conductor over drinks. She just gave me a smile and said “now you know.”
What’s amazing to me is that often they really don’t have to try to follow you. The standard gestures are not some arbitrary sign language but instead have powerful psychological – and even psychic 
– effects. They are designed as they are because they need no explanation. They are intuitive, and often musicians have no choice but to obey them.
Case in point:
My teacher has me learn a Haydn symphony. An 8 bar passage in the 2nd movement had the woodwinds and strings play p throughout. However, back in those days I harnessed the awesome technology of records to help me learn. I would conduct the records for practice. And the recording I had had the orchestra go from mf to p in a decrescendo through that passage. So that’s how I learned it. And boy, did I learn it. I could conduct that mother in my sleep.
When my teacher worked that piece with his own orchestra, he played it as written, p throughout. Now, during this time, I was not just his student, but his assistant conductor. Which meant that I never conducted, but I did move a lot of chairs and drums. I was the orchestra lackey.
During the dress rehearsal, he called a break, then asked me over for a talk. “Why are you dressed like zee bum? Zees eez unacceptable! Where eez your baguette? Always bring zee baguette! Here, you borrow mine. I want to see how it sounds from zee back of zee hall. When zee break she eez over, you do zee movement deux.”
I was terrified. I had never faced a real orchestra before. And there I was, in ripped bluejeans, stained and smelly t-shirt, with a borrowed baton. So I walked up on stage, onto the podium, and faced the orchestra. I was shaking all over. I started conducting.
I don’t remember much, except that I was running on zee autopilot. I was too nervous to turn pages, so I conducted from memory.
OK, short story long, I got to the passage in question. Forgetting myself, I merely thought of the decrescendo. My hands must have subconsciously followed suit. As did the orchestra. They completely ignored what they had rehearsed, and pulled my mistaken intentions directly from my brain. Without really trying.
Wow. Now that’s power!
The happy ending to this story is that they did not, in fact, tear me limb from limb, as I had originally feared.
I can only speak for myself here.
The answer is “sometimes.” It’s not fair to the musicians (but who really cares? ). It’s also dangerous, espacially during a performance.
But yeah, subtle swells and phrasing changes are permissable even during a performance. They can add a freshness and vitality to the work.
What does he do? Why, provide a low-resistance path to ground for the lightning, of course! Especially when he raises that baton.
I’m sure you’ve got your answer by now, but…
Are you under some delusion that an orchestra is a democracy?!?
Seriously, though…
Things run much more smoothly when there is one dictatorial mind that makes the decisions. A debate over every crescendo would be counter productive. As well, the verissimilitude of baton technique is such that it is far more effective and efficient than verbal explanations. A good conductor barely needs to speak during a rehearsal. Run through a piece just once with good baton technique and good musicians will automatically get what they need to get.
I should mention that all this talk of conductors getting hit by lightning is absolutely false. I suspect what really happened was planned by a conspiracy of brass players.
Oh, crap, CandidGamera and Fritz beat me to the bad electricity joke…
Trombone section, we’re watching you…
…and we know they’re not watching you!
What did you have for breakfast? Anaamika?
Sorry, I just had to.
One thing I learned from knowing the conductor of our community orchestra is he spent a lot of time outside of rehearsal negotiating with musicians, smoothing over personalities, and befriending the volunteers. Musicians are a prickly bunch! Even when they’re just playing for fun and have day jobs, they still create a lot of personnel issues. They bicker over who gets to play which part, who’s dragging down the section, and why they should have to come to rehearsal at all when they only play 15 notes on page 6.
The conductor can also become the “face” of the orchestra - if yours has a lot of personality and passion, it will attract more audience members. Putting butts in seats is a huge contribution.
The music part is the biggest thing, though. When you’re playing in sync with your orchestra and your conductor, he’s guiding your actions, you’re responding, the music is flowing – much better than sex. Really good conductors are sexy as all get out.
I can do all that. And I do.
When I’m alone and listening to Alte Kameraden or Stars and Stripes Forever.
[sub]…grumble…damn Sliphorn players…[/sub]
It’s worth noting that back in the old days, orchestras (and they were smaller then) didn’t have conductors. They would follow the lead of the the principal violinist, usually. Sometimes you see period performances delivered this way. Don’t know when the institution of conductor came about – I would guess early 18th century.
There are also conductors who way overdo it. Bernstein comes to mind – there was no reason on earth for the man to be sweating that much. It’s conductors like that who give rise to the notion expressed in the OP, in my opinion. Theatricality=imposture, in this reading of things.
Does the end not justify the means? Bernstein’s performances are astonishing and unique creations.