Release mechanisms come in two types: The first type works by keeping a set of keys out of your reach for some time period (key release mechanisms). The second type works by cutting a rope that is part of the construction that keeps you tied up. (I will call these rope cutting mechanisms, although in none of them the rope is actually cut)
Key release mechanisms have the advantage that they are simple, and therefore reliable. However their safety depends on other assumptions: That the keys fall within your reach, and that you will be able to free yourself, once you have them. After having been tied up for some time, your abilities may be much decreased, so be careful.
Rope cutting mechanisms are more complicated than key release mechanisms. Therefore, they are technically less reliable, but they can be used in cases where key release mechanisms cannot be used, typically in situations where you are totally immobalized, your hands are away from the floor, or separate from each other. I have used rope cutting mechanisms for the rack (see elsewhere on this page), for tying myself to a bed, and for tying myself on a chair.