I've always like those Tri Sports
I think you may be talking about a torque converter. If so, it's basically a shiftless transmission that delivers power & speed when needed. The drive hub ( on the engine)
floats in & out changing the size of the pulley, which in turn delivers the required amount of speed or power. There's lots videos on youtube about them.
Hope I'm not on the wrong track here, & it helps you out.