This is a very good question. Its really something I have never given any thought but now that you did bring it up. When I mow I bring the mower to WOT ( wide open throttle) then I engage the PTO.
However this discussion ends I am impressed with the engineering and durability of the PTO clutch. It takes a real beating. I always wonder how many more cycles are left and purposely reduce the times I use it when possible. And I guess it ain't cheap.
I contacted the dealer and they are going to get me the manuals. I will keep with full throttle to engage deck like they told me. I guess the people who used it before me must have worn the belt out in 4 hours