If you’re mountain biking in the mud through uneven, slippery terrain you need to be ready for the wheels to suddenly move sideways.  This means hovering over the saddle, rather than sitting firmly on it.

What’s the Difference Between Road, Mountain Biking and Commuter Kit?

By hovering the downwards force from your body is hitting the bike at a much lower point – through the pedals and bottom bracket rather than the saddle.  As a result, if the bike wheels slide sideways the whole machine moves sideways with them, rather than tipping over and throwing you off.