Supposedly, it's simple Newtonian physics.
The following is from Wecht's testimony at the Menendez retrial. See:
http://www.courttv.com/archive/casefiles/menendez/transcripts/dec95/
24 Q OKAY. IF THE ARM WERE AS YOU WERE
25 INDICATING, THAT IS, JUST LOOSE AT THE SIDE, AND THE
26 SHOT WERE TO GO FROM THE BACK OF THE ARM IN THROUGH
27 THE FRONT OF THE ARM, WOULDN'T THAT HAVE A TENDENCY
28 TO PUSH THE ARM IN THE DIRECTION OF THE SHOT RATHER
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1 THAN AWAY FROM THE SHOT AND INTO THE CHEST?
2 A NO. I CAN'T TELL YOU HOW THE PERSON
3 WOULD MOVE. THERE'S NO WAY TO KNOW THAT. THE ARM
4 WOULD MOST LIKELY MOVE WHETHER THE PERSON WOULD MOVE
5 THE WHOLE ARM OR WHETHER THERE WOULD BE SOME FLEXION
6 OF THE ELBOW. I DON'T THINK ANYBODY KNOWS THAT AND
7 THERE'S NO WAY TO DO AN EXPERIMENT ON THAT.
8 Q ISN'T IT YOUR UNDERSTANDING, THOUGH, AS
9 A MATTER OF PHYSICS -- I KNOW YOU'RE NOT AN
10 ENGINEER, BUT AS A MATTER OF PHYSICS, WOULDN'T -- IF
11 YOU IMPART A FORCE IN A SPECIFIC DIRECTION, ISN'T
12 THERE A TENDENCY TO MAKE THE OBJECT THAT IS STRUCK
13 TRAVEL IN THE SAME DIRECTION UNLESS THERE'S A REASON
14 TO BE THROWN IN A DIFFERENT OBJECTION?
15 A SURE.
16 MS. ABRAMSON: OBJECTION. WELL, I'M GOING TO
17 OBJECT TO THAT AS IRRELEVANT UNLESS THE OBJECT WE'RE
18 TALKING ABOUT IS PART OF THE HUMAN BODY.
19 THE COURT: OVERRULED.
20 THE WITNESS: THAT'S ONE, I THINK, OF THE
21 BASIC NEWTONIAN PRINCIPLES OF LAWS OF PHYSICS;
22 HOWEVER, YOU SEE, WHEN YOU DEAL WITH THESE THINGS ON
23 A PURELY ENGINEERING BASIS, YOU'RE FORGETTING THAT
24 THE HUMAN BODY HAS MOVEMENT AT JOINTS AND YOU'RE
25 FORGETTING THAT THE HUMAN BODY HAS MOVEMENT THAT IS
26 CONTROLLED VOLUNTARILY OR EVEN INVOLUNTARILY BY THE
27 PARASYMPATHETIC NERVOUS SYSTEM.
28 SO, YOU KNOW, THAT'S THE PROBLEM IN
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1 TRYING TO DEAL WITH SOMETHING LIKE THIS FROM A
2 PURELY MECHANICAL ENGINEERING OR PHYSICIST
3 STANDPOINT. SOME OF THE CONCEPTS, INDEED, ARE
4 APPLICABLE AND RELEVANT, BUT YOU HAVE TO THEN FACTOR
5 IN THE BIOLOGICAL ELEMENT, THE ENTIRE NEUROMUSCULAR
6 SYSTEM AND SO ON, ALL OF THE VOLUNTARY AND
7 INVOLUNTARILY REFLEXIVE ASPECTS OF IT, AS WELL AS
8 WHAT SOMEBODY SETS OUT TO DO IN THIS KIND OF A
9 SITUATION.
10 AND HERE, YOU KNOW, SIR NEWTON AND
11 OTHERS JUST NEVER DEALT WITH THOSE THINGS. THEY
12 DEALT WITH STATIONARY OBJECTS MOVING IN A CERTAIN
13 FASHION WHEN OTHER OBJECTS STRIKE THEM OR ARE STRUCK
14 BY A MOVING OBJECT, AND THAT'S JUST A VERY DIFFERENT
15 SITUATION.