A charge pump is a device that needs a fixed frequency input, at which point it then activates, and is typically used on BOBs to ensure no outputs are activated until the charge pump is active.

The problem with a parallel port, is the pins may switch on/off during power up/initialisation. By using a charge pump, it means the parallel port driver has to be full control, and generating a fixed frequency output, before any outputs get activated. It also provides some safety in the event the computer crashes, as it should cause the charge pump to fail and deactivate outputs.

If it's just the coolant switching on, I wouldn't worry too much. The big concern is if the spindle happens to activate, or anything else that could cause physical damage.