At X tension it is released vs w/o the mechanism they must release optimally at around X(give or take Y). When trying to control it themselves, they attempt to reach X tension but in their attempt to do so they wince since you must purposely release the tension instead of it just having it go on its own. This wince is a reflex to an act you are trying to accomplish (purposely release the tension). This wince is bad for control.

When the surprise mechanism goes off, you still do you natural reflex, but the difference here is that the reflex occurs after the release (a consequence of the act instead of bracing for it) so that miniscule movement should not affect the trajectory as the arrow has already been released.