This 4-inch blade is attached via a hinge to a wooden handle with a slot in the center, into which the blade can be folded. With a click of a switch, a spring thrusts the blade outward, where it locks into place until it is reset.
Releasing the blade is a free action, and resetting it is a move action. The handle must be in your hand for the blade to be released; placing it in your hand requires the same type of action as drawing a weapon. A spring blade is easy to conceal: an observer must succeed at a DC 20 Perception check to notice the blade hidden within the wooden handle, and you gain a +4 bonus on Sleight of Hand checks to conceal a spring blade on your body.