Every water/methanol injection system needs a way to route where fluid goes, regulate how much volume is delivered, and to time delivery to the engine's need for it. Three types of devices are available for these purposes:
The simplest are check valves (which are spring-loaded and open passively under pump pressure). -useful for stopping leakage and drips but impossible to control independently.
Whereas, flow control solenoids are electrically actuated. Switch on 12V power to immediately open them 100%. Switch off that power for instant full closure. It's all or nothing, but actuation conditions and criteria are up to the tuner.
The most powerful actuators available are RACE valves. These are pulsed and tuned exactly like fuel injectors.
RACE valves operate very similarly to OEM technology (but at 200psi):