It's not all that easy to explain and the pictures in the manual are not very good. In practice, you lower the hydraulic lift system and the
PTO coupler at the forward end of the transmission comes down on the right hand side in front of the deck's gear box. When you drive slowly forward, the coupler gets on the splined deck's shaft like any other transmission. In fact, I sometimes have to shake the deck a bit to have the locks engage completely when I haven't driven perfectly straight over the deck, but the transmission always gets straight in first time. The only trouble is there are three grease fittings on the driveshaft which are supposed to be lubricated every 25 hours ...