The slipped bowline is useful when the load on the loop is constant and you need a quick release.
A. Tie a bowline.
B. & C. Make the last tuck of the knot with the end doubled in a bight.
D. Shape the knot and pull tight in order to prevent slipping. The knot can be quickly released by pulling the bitter end.
Original materials used with thanks to Irene Rodriguez and John Bennett