Weighting and deweighting your dirt bike is important for techniques -jumps, obstacles, ruts, clearing gaps, hill climbs, and finding traction. In many cases, if you are sitting on the bike in rough terrain or tackling obstacles, your body is just dead weight on the bike, and becomes part of the problem. If you stand then weight and deweight correctly, your body weight becomes part of the solution.
Originally weighting and deweighting was a trials technique, where the rider's weight compresses the suspension to gain traction or climb over obstacles.
You push down on the footpegs to compress the suspension. As it rebounds, you also bend your legs to take your weight off the footpegs to lighten the bike.
If you combine this with clutch and throttle, you get a lot of drive because you temporarily flatten the rear tyre and get a bigger tyre patch. In most cases, you will weight the front and rear suspension equally, but you can put more emphasis on the front for techniques such as pivot turns and wheelies.
Timing this can be difficult at first, so often it's best to not worry about the clutch. But once you get the hang of it, then start to combine a controlled drop of the clutch too as this makes a big difference for more advanced techniques.
Weighting and deweighting the bike works great for many low speed techniques such as hopping logs, pivot turns and rear wheel hops - but it's also very effective at faster speeds for jumps,
Many of these are low speed techniques, but weighting and deweighting is also very effective at speed for jumps, finding traction on faster hill climbs, and clearing gaps and ruts.
Weighting the bike at the right moment can dramatically increase how far your bike jumps if you need to clear rough ground after the jump. Conversely, if you deweight at the right moment, you minimise your air time and can get the bike back on the ground and accelerating sooner.
If you see a gap or series of ruts ahead, weighting and deweighting the bike allows you to clear these if you have enough speed. With ruts, even if your wheels are hitting them you are less likely to be deflected if your weight is not on the pegs.
If you are in rough terrain, you can still weight your bike even if you are 'paddling' the ground with one foot. Where possible, keep one foot on a footpeg so you can still weight and deweight the bike as needed.
Remember start with easy practice drills on flat ground to get your timing right, then work this into all the techniques mentioned. Good luck!
to watch a training video about weighting and deweighting.