Chris S said:
It would be pretty darn difficult to fully compress and hold the rear suspension while you check chain tension, anyway.
It's actually quite simple.
Stand on right side of bike, lay across it and put all your weight on it, using your toes to keep yourself upright. This will allow you to see the chain and check the play with your hand while your weight is on it. Of course it will compress a little more than that amount when you land a jump, so you want a little bit of play when your weight is on it.
IMHO, a little too loose is better than a little too tight, as that will wear the bearings, the sprockets, chain, etc. Especially when jumping.
See attached picture for reference, keep in mind that the bike in the picture is on a center stand, but you get the idea. There is a long rubber slider on the top of the swingarm near the pivot for a reason.
