The difference is what actuates the release. In a standard release, you use your fingers (index or thumb) to release the arrow. The problem is that fingers are sensitive and prone to twitchy behavior, which means anticipating the shot leads to premature releases (which we call “target panic”). Your eyes see the target and it tells your twitchy fingers “release the arrow NOW or you’ll run out of time!”. Releasing with the fingers is a conscious decision that eventually becomes an unconscious one due to anxiety, leading to premature shots. A surprise release device (also called a “back tension release”) uses a different muscle group to release the arrow. You see, with a surprise release device us archers are taught to use the muscles in our backs to pull the string (specifically the rhomboid muscle). Flexing the rhomboid is a slow and steady process, since the rhomboid isn’t a twitchy muscle. So what ends up happening is we squeeze our rhomboid muscle which at some point along that flexion of the rhomboid, it triggers the surprise release aid to release the arrow. We don’t consciously know when flexing our back muscle will trigger the release, so we can just focus on the target.