Aqualisa shower?

why is it when i swtch off shower from the small round control unit,the power seems to go off or at least the red light does but the water keeps coming out the shower head not at full speed but say half speed, i'm sorry we have just moved into this house, no details of the shower were left for us so i dont know the model, the only way we can stop the water coming out is to switch off the power from the mains.