LOL. Well, I woulda too. In fact, if I had a tennis ball, I probably would still try it… 
Here’s why it won’t work.
In order for air pressure to open a car door lock, the car door has to be air tight. It isn’t. I’m a notorious do-it-yourselfer living in a small town in which every other guy is a shade tree mechanic. I’ve taken apart or helped to take apart numerous car doors. They ain’t air tight. And even if they were, the amount of air in a tennis ball won’t displace enough air inside a car door to make any difference. You can poke an air compressor hose into your door lock, and it won’t unlock the car. The stuff just doesn’t work that way.
Is it coincidence that the two people here testifying to the veracity of this are Brits? I’ve definitely heard that this can work (though no firsthand experience) on some older cars, always with the half-ball, like Fridgemagnet suggests. Could it just be ‘BS’ with North American cars, and true with certain European models?
Somebody stop Galileo spinning in his grave. :dubious: “My theory ses it can’t work so it’s bullshit and not worth doing the experiment.”
Fighting ignorance doesn’t come from assertions but from facts. We have a video and two contributors as evidence that it can work in some circumstances and one experiment where it did not work. We need more evidence – either from a reliable source or from more experiments (maybe a repeat on the cars where it was seen to work in the past?) before we can reach any conclusion on how it might work. Once we have a theory as to how it works we can make predictions as to which cars it will always work on and which it won’t.
Someone has to send this over to the Mythbusters!
They will only do it if it involves blowing the car up with a tennis ball. 
I used to think it was bullshit that somebody could open a Kryptonite lock with a Bic pen.
However, I still think it’s bullshit that you can open a car remotely with a cell phone.
My Best guess on the matter.
The burst of air pressure in the lock will not under any situation cause the mechanical portion of the lock to move allowing the vehilce to unlock.
That being said…
Most vehicles that have a remote or electric locks also mount a switch on the back of the lock. this switch will lock and unlock the other doors on the vehicle when a key is used…
My best guess without going out and pulling door panels. Some vehicles have a flaw in their design which allows a burst of air to effect the switch mounted on the back of the lock. Also, many european vehicles will use vaccum hoses to lock the car doors.
It could be a vaccum style system has this flaw. It would also explain why it seems to be more common in europe.
my $.02
Osip
OK, Let’s talk about this
In order for the key to turn a series of tumbler pins have to rasied to exactly the correct height. For my example I am going to use the lock on a Volvo 240 since I know the particulars from memory, and I am lazy. There are seven sets of pins. Each pin can be short medium or tall.
Short =1
Medium =2
Tall = 3
A key code for this car might look like 1322132
So how do the air in the tennis ball know how far to raise each pin? 
How does it keep from rasing the pins to 312232?
Now just for shits and giggles let’s assume by smacking the tennis ball on the door that the pins are raised to the correct position. Now the lock cylinder can turn, which will pull/push the rod that actually unlocks the door. On some cars the key turns counter clockwise, on others it turns clockwise.
So assuming that the air in the tennis ball is smart enough to raise the pins to the proper position, How do it know which way to rotate the lock? :rolleyes:
I think your tennis ball would have to be loaded with extra smart air.
Please share your hypothesis on just how the air in the tennis ball is going to accomplish these tasks.
After you can solve those two riddles, we can move on to just how the tennis ball full of air can exert enough torque to move the lock internals. :smack:
Some older European models have vacuum operated locks
Note that the explanation seems to be that the air pressure doesn’t turn the lock tumbler, but (in affected cars) operates the separate override mechanism used for remote unlocking.
Just to clarify, you don’t have to raise the pins exactly to these levels. You would just need to raise the top pin above the shear line, while keeping the lower pin below the shear line and turning the lock. Note that a tennis ball would probably not accomplish this task, but lockpicking devices like rakes and picking guns do.
I own just such a car, and I cannot see any way that this could make a difference. As RIck said, the pneumatic central locks are a closed system, they have to be, or how else could it activate the locks?
Half the time the stupid thing is broken anyway, probably why they don’t use them any more.