To void the bowels, expel a turd or the like. To defecate. Of military origin, the phrase refers to the process of taking the strain off the spring system in one's rifle. The similarity is drawn to the subsequent release of energy and altogether more relaxed feeling afterwards.
"2 Section. Your patrol was due to start 2 minutes ago. Why are you still here?" - "Jimmy had to away and ease springs. We'll be off shortly, sir"