I wanted everyone to know that I have after much painstaking research found the secret to water knapping heated stone. We all know to heat the rock to at least 500° F, but that is where it departs from the usual dropping of water on the blank that we have all heard about. I have learned that by brushing the water on with a feather (some say an eagle feather, but that is bogus and against the law to boot - any feather will do), you can gain control of the chipping. I dip the feather in water, but instead of putting the feather against the rock completely wet, I shake the feather out so that only a trace of water remains on it, then I brush the biface where I want the flake scar to appear. After about two minutes of doing this, the rock must be heated again. I have been accomplishing that by keeping a turkey roaster full of sand on 500° and shoving the point back into the sand for a few minutes so it can regain full temperature. It works like a charm.
I must apply this warning however - do not try this at home, work, on the moon or anywhere without a full welding suit and hood. No exceptions. This technique only works on April 1st and no other day of the year.