spring-loaded 
adjective
Containing a compressed or stretched spring pressing one part against another: a spring-loaded clothes peg...
- The doors are spring-loaded so that, at the point of being halfway closed, the spring will take over and slam the door shut and hold it closed.
- In the market I found almost everyone sold heavy, serrated, spring-loaded metal rattraps.
- This knife was like a switchblade, except that the blade was spring-loaded to shoot straight out of the handle and lock when you pressed the release button, rather than opening sideways.
