Twitch vs. Strip
Marty G,
It really depends on the area you're fishing and the prevailing type of bait, including what type of shrimp. In Andros, for instance, the bones (according to my guide) are feeding on mud minnows or bigger shrimp. Both these make longer, slower swimming motions to escape. It might start off w/ a quick strip to get the fish's attention, but then you want to strip slow and long, about 1-2 feet per strip. If the fish is following but won't eat then you let the fly drop and watch for the take. Usually when you strip again it's fish on!
This is a sharp contrast to areas w/ lots of turtle grass bottom, like Honduras or the Cayman Islands (where I fish). There you're dealing w/ grass shrimp as the dominant prey and they're smaller and swim slower. Bigger, faster strips will spook fish there. You want to twitch it, maybe an inch or so at a time, in an erratic way. This accomplishes two things, 1) it keeps the fly in the zone -- which is important in thick grass since the fish has a narrow window of vision around its head, 2) it won't spook the fish. "Pinky-pinky". That's the way.
Ok, crabs: They need to be fished with good sight-fishing conditions. It's a strip and stop retrieve. And, yes, if you move a fly just before a fish eats it, they'll spook. That's a given. Prey doesn't do that (I'm assuming). Basic rule of thumb is watch the fish's reaction. If you cast the crab and it sees it falling and swims over to check it out, you don't need to move it until a) the fish eats it or b) it starts to swim away from lack of interest. Bonefish can flat out SEE and I've watched them study a fly for seconds before finally tailing on it hard. Be patient and watch for the take.
Now, one final thing, if you're fishing w/ a guide he might have you strip a few times after each cast and then stop. This is often to position the fly correctly or (more likely) to remove slack from the fly line. He might also have you strip sooner than you expect for a fish on a crab fly in anticipation of the bite because he can see slack in your line and knows if the fish eats on a slack line he'll spit it before you can come tight. So, you can't always go by what a guide tells you to do as gospel for stripping a fly. Often he has other considerations.
Hope this helps and tight lines,
Bonehead the Angler