Kelvin,
I am not sure if this will answer you question or not, but Yanmar has some models with a power shift trans. This allows the operator to stop the motion of the tractor, either forward or reverse and still have power to the PTO. Essentially, the tractors have two transmissions. The first is a standard transmission that requires the clutch to be depressed, which stops the PTO. The second is the power shift, which does not require the clutch to be depressed. This allows the user to maintain PTO speed while shifting between F1, 2,3,N, and R.
I find this very useful while mowing. I can drive forward, stop and reverse into tight areas, without losing the power to the mower.
These Grey Yanmar models can be identified with their model number. Any model that ends with a XX10, XX20 or FX is a power shift.
I hope that this helps,
Cameron