inverted flight
RC models have no teetering hinge normally, so the head is rigid (hingeless) except for the blade attach bolt that also serves as a lead lag hinge. The collective pitch is set for several degrees of negative pitch in aerobatic models, this allows sustained inverted flight.
For a full size helo the collective lower stop is set near 0 degrees for autorotation. The 5 degrees or so of negative pitch required for sustained inverted flight would be a problem for the pilot in an autorotation emergency. Just one of the many problems to consider.