Which witness was this, Walt, and exactly where did they say it?
The witnesses who reported seeing the man pointing the barrel up and shaking the pistol, and discarding the shells were the Davis girls. Domingo Benavides described the man that he saw shoot Tippit as walking away and tossing a shell ( ONE SHELL)in the bush as he walked away. The spent shells that were recovered at he scene Were found scattered about, and not in a cluster as would be expected if they had been extracted from a S&W.
So it's not one witness, as advertised, but a combination of the testimony of three different ones. Good to know.
Anyway, Benavides saw the gunman remove and throw two cases, not one. Then again, he remembered that he heard three shots, when there must have been at least four, so his counting may be off.
The shaking described by the Davis girls doesn't solve your problem. Their description implies two different possibilities to me:
First is that the Davises saw the guy doing what might be called the 'combat crane close.' This maneuver involves closing the cylinder by abruptly rotating the wrist of the hand holding the pistol. If you remember old-school cop and detective shows, you've seen some actor do it at least once. Sometimes, the cylinder doesn't engage the frame and bounces back, requiring a second or even a third shake to get the thing closed.
Second is where the rim of one or more cases slips past the ejector star and fall back into their chamber. If that happens, one easy way to get them out is to shake them out. In any case, if you are having to shake the cases out of a revolver, the ejector isn't doing the job and the cases aren't coming out normally. Oswald's pistol was originally chambered in .38 S&W, but the chamber was rebored to fire .38 Special. .38 S&W is slightly larger diameter than .38 special. The modification of the LHO pistol can cause .38SPL cases to permanently swell or even split during firing, which would definitely make it harder to unload normally. And it's a situation liable to cause someone to pick out cases onesey-twosey.
BTW, I don't see either of the Davis girls saying that the shooter was pointing the barrel upwards when he shook the gun.