Not having the schematic of the
PTO functions - I would speculate the electro magnet is energized all the time the tractor is running WHEN the safety switch(s) are made (far as I know there is only one under the seat - that works all the way out or all the way down in to make the circuit complete to the magnet).
With the safety switch 'made' the circuit is complete and the magnet will hold onto the handle and pull up the arm that really engages the
PTO.
When mine worked - I never had a problem with it staying 'engaged' regardless of what bumps I hit - lawn mowing or mowing with the 5' brush hog. Maybe your magnet is either dirty (as was mentioned earlier) or getting week, time for a replacement (easy to get to on my 88', just pull the side cover and one bolt to remove the arm and a cotter pin in the rod, disconnect a plug and it is in your hand).