i'm in a rush but sent this to somebody and then thought maybe posting it would help other somebodys.
anyway, i need to do a video on end-snap and how to prevent it. there are a lot of little, subtle nuances to keeping that phenomenon from occurring. quick overview though. first of all, thin the ends FIRST. the point end doesn't matter much as far as shooting flakes straight down it because since it's normally tapered to a point anyway from the sides, it's pretty easy to get rid of thickness with hits that are less likely to set up the dangerous vibrations. but both ends should be thinned as well as possible before the middle, and most knappers get their bases all in order way too late in the reduction process. i tend to finish my bases before the rest of the point is finished and then make the blade and tip of the point work with the established base. but i do my bases too late too, but i have no problems at all hitting on the ends.
when i hit on the ends, i make sure to have no force vector angled downward any more than absolutely necessary. and i "pinch" hold the platform together with a support finger under it and thumb on top to counteract the slight downward angle needed to get the concoidal fracture going straight on a plane with the face. my goal with those flakes is almost always to hinge them out flat as far in as necessary to thin, and then come back from the sides and cut off the hinges with more flat flakes. trying to hit beveling flakes from the ends is extraordinarily dangerous and i just never do that after i've thinned the middle much. get the ends close to right where no short beveling flakes will be needed before you thin the middle.
ok so the rest of the story is support. i tend to jam the opposite end of the piece into my palm or my leg, depending on the length of the piece, and then hold the platform together as previously describe, and then have my hand along the far edge of the piece, just in contact with the edge to dampen vibration. but what i DON'T do is hold, press, squeeze or otherwise grip anywhere on the work in between the two ends. no bending forces in the support at all.
oops, forgot something. swing accuracy is a major risk factor. you want to catch only a small platform contact area. AND you do not want a weak platform to fail, but you darn sure don't want an overly strong platform or to catch too much of the edge or otherwise "overstrike" it. that WILL initiate bending force and vibration that will overcome your support and break the piece anyway. along those same lines, you have to get instinctive from repetition at understanding almost exactly how hard you have to hit to take the flake you need, but no more. putting unneeded extra force into strikes is as bad or worse than putting too little force or speed into strikes. but a key key point is that the flake needs to initiate cleanly and rapidly. if for whatever reason it doesn't, all that force gets transmitted to the whole piece instead of just the flake, and that's vibration-city.
most people seem not to understand how little vibration it takes to make a piece fall in half. it's like the vibration magnifies itself or something. it's particularly bad when the ends are heavy, or there's not much convexity.