in my I14 days we used to have a telescopic type mast setup, such that light days = big rig, heavy days = small rig. you would reef the mainsil from the bottom. i.e. roll and zip up the bottom 700mm and you would have two sets of tack/clew points. one of our guys thought it would be easier to leave mast at full height and just run the main up the mast so the head was about 700mm from the top. end result was ( in this order and about same timing as you are reading)
1, round top mark
2, kite goes up to mast head
3, sheet comes on
4, top of mast comes off
it was plain and simple to see that it is the leach / sheet tension of the mainsail that holds the mast up under spin
p.s. and we were all using diamonds to the mast head, swept back at about 45 degrees.