I just realized there is a gap in my knowledge of basketball. Let’s fight my ignorance.
What would happen if a basketball player jumps during a free throw attempt?
Does it matter if it is a forward jump or not? Or if it is a tiny little jump? If it’s not allowed, but a player tries it and makes the shot, do they have to retry it or is the attempt nullified?
Would this convey any kind of advantage? Imagine someone jumping forward and shooting at the last possible moment before landing.
Is it physically possible for someone to dunk the ball from a standing jump from the free throw line?
Let me preface this by saying that I’m not - by any stretch of the imagination - a basketball expert.
A forward jump is certainly not allowed. In fact, some free throws have been ordered reshot because the shooter’s foot was over the line. I would imagine that any free throw attempt where the feet leave the ground is shot over.
Is it physically possible to dunk from a standing jump at the free-throw line? I can’t see why it would be physically impossible.
I believe the rule is that you can’t cross the line before the ball hits the rim- and if you jump forward or attempt (and then fail) to dunk it, you’ll have crossed the line. Incidently, this rule is broken quite often without penalty- Shaq in particular is guilty of this one.
Also, I’m pretty sure the shot is completely nullifed- no redos. But then, I’ve never seen this called in a pro game (I had it called all the time on me when I played in middle school, though) 
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I don’t believe anyone could jump 13.5’ away from them and and still be 2 or 3’ off the ground at that point (from a standing position).
The freethrow line is 15 feet from the backboard. The hoop is 18" in diameter and 10’ off the ground. I know the standing broad jump used to be an Olympic event, but I couldn’t find a record for it.
The only reason players jump when shooting during the course of the game is because there are big defensive players trying to block their shots. Without such opposition, you’d much rather have a stable base to shoot from – i.e., feet on the ground, not in the air.
Also, free throw shooting is all about repitition, practicing making the same motion over and over and over again to consistently sink the shot. Jumping just adds more variables to deal with, which would inhibit developing that consistency.
In short, it’s a really bad idea, which is why no one does it.
In terms of shooting, probably not, but it might grant an advantage in rebounding, since the shooter would have a big edge on the guy trying to block him out. I expect that’s why it’s not allowed.
Yeah. Players dunking from the free throw line need to be on the dead run. The last time I saw this done was by Brent Barry in a dunk contest, and he needed to run the entire length of the court before jumping from the foul line.
Sorry, I meant to add this about the rule: Israfel is exactly right. The shooter can’t cross the line before the ball hits the rim, and the players trying to rebound can go in as soon as the ball is in the air. Both rules are definitely broken all the time. Younger kids in organized leagues are allowed to jump while shooting, or at least I was until a certain age.