I believe it's surprise for the body rather than "surprise" in the standard sense. The idea is that you're doing a single motion that triggers a release rather than that motion plus an anticipatory release of the fingers/trigger pull which can cause the body to jerk. More of a "get ready and it releases on its own when it's in the right place" vs "get ready, then fire".
Spazzout22 t1_ixzglzc wrote
Reply to comment by leadchipmunk in TIL that many pro archers use surprise release mechanisms to prevent themselves from anticipating and reacting to the impending impact. by broogernavn
I believe it's surprise for the body rather than "surprise" in the standard sense. The idea is that you're doing a single motion that triggers a release rather than that motion plus an anticipatory release of the fingers/trigger pull which can cause the body to jerk. More of a "get ready and it releases on its own when it's in the right place" vs "get ready, then fire".