We had one for 30 years. Actually, the YM135/155/165 are all pretty much the identical tractor. They all were about the same size, and had a two cylinder marine vintage engine. Tough as a nail. The engines were thermosiphon-cooled tractor and only came with a manual 3 speed forward x2 range transmisson. It is a tough tractor, and overbuilt for its size. With decent care they go forever. Yanmar made a nice loader for them, too - and even put a full length loader frame under it to farther protect the tractor.
The PTO is the most basic type: a transmission-driven PTO. So if you are in gear and hooked to an implement that will be used when you are moving - like a tiller or snow blower - you stop, push in the clutch, select a gear, engage the PTO lever, and take off. The PTO will not begin to turn until you begin to go forward. It won't reach rated speed until you get to the rated ground speed in that gear. There are two PTO speeds: 550 and 770 rpm which are selectable with a shift lever.
For applications where you don't need the tractor to be moving - log splitter, backhoe, post hole drill...things like that - the PTO works like any other. You put the trnsmissioin neutral and the foot clutch works the PTO very nicely. The throttle regulates the PTO speed.
Use 50/50 ethylene glycol antifreeze and water. Clean the radiator & screen religiously....not much else to go wrong.
rScotty