In many full scale biplanes and models as well. the center of mass is slightly below the actual rolling axis in straight and level flight. When you roll from upright level flight to inverted, you are "lifting" the center of mass "up" and around the outside of the roll. As soon as you depart level inverted flight, as the center of mass departs top dead center, it acts like a pendulum seeking the lowest point and therefore gives a kinetic "boost" to the rate of roll. In a full scale airplane this would manifest itself as lighter stick pressure for a given roll rate (initially). For us holding a transmitter, it could mean less stick displacement for a given roll rate. Hope this didn't sound like total BS but if I took you for a ride in a Stearman or Great Lakes Bipe you would see what I mean.

BOB