The weld control logic monitors all input information and stores all programs and initiates all necessary commands to complete the required functions. In simple terms when the input signal for a weld is received from the foot switch or other input source the control logic starts the squeeze/weld/hold/off sequence as programmed.
It has the ability to maintain constant voltage or constant current or employ current steppers, temper, forge and many other functions. The type of control and the complexity of your input program decide what the actual weld program will be.
Reference: RWMA - Resistance Welding Manual 4th Edition