The simple answer to "how concrete pumps work" is, a concrete truck discharges its concrete into a hopper on the back of the pump truck. The hopper has a mesh grate the concrete falls through which prevents any large rocks or chunks from plugging the pump truck hoses. The hopper also has an auger that churns the concrete keeping it liquid and flowable.
Once the concrete is in the hopper, it gets sucked into a valve system in small intervals or strokes. As some of the concrete is being sucked into the valve, the concrete before it is being simultaneously pushed through the concrete pipe on the pump truck boom until it reaches the end of the concrete hose and placed where you need it.
Some concrete pump trucks have different types of valve systems, s-valves, ball-valves, but basically that's how a concrete pump works. As long as the hopper stays full and there are no blockages in the concrete hoses, the process of pumping the concrete goes quite smoothly.
Having a good concrete pump truck operator is also very important. Their experience and wisdom is key to handling any problems that may occur, fixing them, and keeping the pour going.