OK, I found some more info and think I get it now. Yanmar's HMT is a more conventional variable drive with a planetary gear, hydraulic pump, and hydraulic motor. But the i-HMT is indeed a very different design and does not include a planetary gear - or at least it doesn't look like it does to me. Note that in no case am I talking about the planetary gear in any differential. That's a totally different thing. This is just in the transmission.
I'll attempt to explain how it works. Wish me luck..
The input shaft is direct coupled to the pump body, and the matching swash plate is stationary and variable. When the input shaft spins, so does the pump, and based on the swash angle, it pumps fluid. Just keep in mind that it's the piston and cylinders that are rotating, NOT the swash plate. I think it's much more common for the swash plate to rotate and for the cylinder and pistons to be stationary.
But the pump body is also the motor body. The pump pistons and swash plate are on the front, and the motor pistons and swash plate are on the back. The motor swash plate is at a fixed angle and drives the output shaft.
So when the input shaft rotates and there is an angle on the swash plate, both the pump and motor bodies turns, and the motor pistons drive the motor swash plate, but in the reverse direction from the input shaft.. At max pump swash angle, the motor swash turns at the same speed as the input. But because it turns in the opposite direction, the result is that the output is stationary. At zero swash angle, the motor is stopped, so it's swash is rotating at the same speed as the input. The input is effectively direct coupled to the output. So across the operating range of the pump swash, the output shaft speed is equal to the input shaft speed minus the motor speed.
It's really pretty clever, but I'm not sure how it compares in efficiency and other aspects to a planetary variable transmission. I'm sure there are a variety of pros and cons - I just don't know what they are.
