Hello Jason, this is a great question.
Phase control is generally referred to when discussing AC to DC power conversion.
Devices called SCRs (Silicon Controlled Rectifier) are used in this process of power conversion. So before I talk about phase control we need to discuss the SCR briefly. It is like a diode as it only conducts in 1 direction. However the SCR does not conduct automatically like the diode does. The SCR requires a Gate pulse with respect to the Cathode when the the main power terminals are forward biased. If SCRs are used in an AC circuit then the Gate pulses must be timed or synchronized in order for them to conduct at all. PHASE Control means that these Gate pulses are shifted up and down the half cycle of AC across the SCR. If the Gate pulse comes in late in the half cycle then the SCR only conducts a short time and little current is passed through. If the Gate pulse is shifted up early in the half cycle then the SCR will conduct a heavy current. In this case it is the shifting of this Gate pulse that is Phase Control. Synchronizing circuits, current sensors and pulse amplifiers all work together to provide the Regulator electronics the means to do phase control, ok?