Having flown R/C helicopters inverted, there are two differences that are obvious. Firstly the blades do not cone to anything like the same extent, and secondly the clearance between the blades and the tail boom is relatively greater.
They also have about ten degrees of negative collective pitch which helps, but the setup of the collective would be interesting on a full size - it would be in the middle and if you raised it you would lift off, but I hate to think what would happen if you lowered it!