Hot water is taken from the cylinder and the cold water from the header tank into the shower unit which contains a pump which boosts the flow rate to give you a more powerful shower. There are no heating elements inside a power shower, so a much smaller cable is required for feeding the shower pump only. A power shower can provide you with up to 14 litres per minute of heated water.
Please make sure that you check the minimum requirements for 'head room' with regards to your hot water header tank.
Both types have separate temperature and flow control and an adjustable temperature limiter that can be over-ridden if required.
The thermostatic model provides additional automatic control to keep the temperature stable through peaks and troughs of water pressure levels.