it stops the flow of water by way of a gate dropping as the handle is turned clockwise which is connected to a threaded shaft that the gate travels up and down on. over time the gates themselves accumulate calcium deposits and usually dont stop the flow of water completely when you finally need to use it. then while tryin to tighten valve even more to stop flow of water the threaded shaft, which is brittle after years, often breaks leaving the valve in an almost all the way closed position permanently. next move is to replace gate valve (junk) with a ball valve.