Funnily enough, unlike on a road bike where you might get up out of the saddle to climb, it's really important to stay on it on a mountain bike when climbing on a loose surface, or your back wheel can spin out.
There's a technique known as "nosing" where you keep a bit of weight on the nose of the saddle when climbing on a loose surface while still being forward enough on the bike to stop the front wheel lifting. Hurts mind you. ;-)
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