It is basically an outboard plus a mounting system that bolts to the back of a full height transon with no cutout. It achieves a similar effect as mounting an outboard on a Gil bracket.
There are some interesting articles, and some pictures, in the reference section of the Continuous Wave Boston Whaler web site:
Cut Transoms and Brackets
As far as I know, the SeaDrive units haven't been in production for quite a while now, although spares do seem to be available.
I don't know of one on a RIB, but various people have use standoff brackets with varying success. The main problems that I have heard of are fore and aft trim with the weight of the engine moved back, and mechanical failure due to the tougher than average use that RIBs tend to get.