This is a direct quote from NHL Rule 24:
"The spin-o-rama type move where the player completes a 360° turn as he approaches the goal, shall be permitted as this involves continuous motion."
Basically, the player gets some leeway. As long as he doesn't turn around and go backwards, do a loop rather than a spin, or completely stop the puck for more than a split second, the shot will be counted.
I think goalies should just go for the feet in this situation. If you can react in time, just go for the trip. Best case, the referee judges you're going for the puck and the shot is no good. Worst case, the player gets a redo, and he probably won't get so cute the next time.
"When an infraction worthy of a minor penalty is committed by the goalkeeper during the penalty shot that causes the shot to fail, no penalty is to be assessed but the Referee shall permit the shot to be taken over again."