Miles covers body position and staying loose on the bike.
Arch your back slightly. Don’t try to stand up as tall as possible. Aim for slightly soft knees, a slightly arched back and soften the arms down to the ’bars.
It’s incredible how smooth elite off-road riders can look as they manage their bikes on rough terrain.
Possibly the biggest single technique that allows this is the way the rider stands on the bike and uses their body to be balanced, efficient, work with the terrain and not be a passenger.