You might have been 'treated' to electro-stimulation by your physio. Using this small, rechargeable unit, you attach electrodes to your skin which stimulate your muscles to contract at varying strengths, controlled using the handset.
There are two main benefits: to aid recovery from injury by building up muscles that your body has been under-using; for example small glute muscles which often become underactive leading to running and cycling injuries. It can also be used during turbo workouts to overload muscles.
Seeing and feeling your muscles jump without willing them is odd at first, and if you ramp up the strength of contractions, particularly in injured or under-trained muscles, it can make you wince and leave you with soreness.
We found it useful for working small muscles that, consciously, we'd struggle to 'switch on', and that helped us with injury rehab. If you have a tricky to treat injury or are keen to condition known areas of weakness, this is worth considering, but bear in mind you could buy 15 physio sessions for the price.