 When the pressure plate has been pressed down approximately 2.5 cm, a single retaining ball bearing flips side-ways into a recess.When the ball bearing flips side-ways, it frees the striker (held under tension by a coiled spring beneath it) which snaps upwards into the stab detonator, thereby detonating the explosive charge.An Elsie mine is constructed mainly from plastic, which makes it difficult to detect.