You may be expecting the rotor to spin up in lock-sync with the engine.
That's not quite what happens on a turbine helicopter.
Turbine helicopter engines (turboshaft engines) have a turbojet engine inside them that makes thrust. This engine has a shaft of its own - so that its compressor blades are connected to its own turbine blades. The turbojet's thrust acts on a second "stage" of turbine blades, which are on their own shaft. Those connect to the helicopter rotor.
So the "internal" turbojet engine can be started and idled, without making enough thrust to spin up the second stage. Turboprop engines work the same way, and you'll see where they are "barely turning over" at times. That is the idle thrust working on their rather free-turning mechanism.
Some helicopters have a rotor brake so they can keep the rotor from spinning slowly like that.
Closer to home, that same effect occurs in your car's automatic transmission. The engine is able to turn at idle speeds while in gear. That's why you have to hold the brake constantly, the engine's "thrust" will creep the car up to about 5 mph if you don't.
Both of these are a hydraulic coupling - transmission fluid in the car, and jet engine exhaust air in the helicopter/turboprop/turbofan.
Once you advance the throttle to meaningful amounts of power, this "slip" becomes insignificant, and power is transmitted through just fine.