I picked up some chopper work for a bit of fun a few weeks back and came across this.
Bell 206B - Allison C20 and so did the origional straight L Longranger (not L1 c28,L3/4 c30).
Anyway in the longranger it has water meth injection, about 2/3 water and 1/3 meth.
Operation is essentially the same as fixed wing but for different engine conditions. Basically pull collective to engine temp, limit but not the TQ limit, hit the meth and the above effects of lowering the temp and greater mass flow, The temp drop is between 50 and 90 degrees celcius depending on conditions.
The temp drop allows you to pull more collective (more fuel) back to the temp limit and hence more TQ (higher collective position) and get you off the ground.
Works well when you need it, essentially fire it off, increase TQ get airborne RFN and once through translation and into climb lower power and WM off.
Thats how it works in the fling wing world.