that switch was a PAIN!
you pretty well have to hold the drip firmly, and even then sometimes the machine will switch off and on as you move the cleaner.
I should have also mentioned there was also the normal 3-position rocker switch on the handle front as well, marked ATT/HI/LO. Flipping the switch up to ATT overode the grip switch and turned the motor on for attachment use. The middle position is HI and allows the motor to run on HI speed when the grip is squeezed. Flipping the switch to LO gives you low speed when the grip is squeezed.
I believe that some recent Rainbow models had a similar switch arrangement for the power nozzle.