I think, with Reg, that in this case it is a sign of trust. Other body language plays a part.
Sometimes a cat on it's back with feet extended is a defensive move. Kind of "Make my day" jump right in here so I can hold on to you with my front claws and rake you with my back claws. In a serious cat fight the bottom cat may have a distinct advantage. AND, cats and kittens play fight and will do this to each other as well as to you, and it can sometimes get a wee bit out of control.
We need a picture.