The shift. When the simulation has stopped:

• holding shift starts simulation and allows for dragging around the cloth/hair.
• releasing the shift, stops the simulation.

When the simulation is running:

• holding the shift allows for the dragging of cloth/hair.
• releaseing the shift stops the simulation.

It would be good if the shift remebered the state, so if the simulation was already running, the shift returned to that state. Or have another key (maybe the caps lock?) that could be used when a simulation is already running?