Continued from part 1 - Standing position.
The one advantage of sitting down is lowering the center of mass, which allows you to lean the bike more into the corners. Also, you could reach down and dab with the inside foot to correct for balance during tight turns. That's why, when the going gets tough, most beginners tend to sit down for that feeling of safety but they do give up quite a bit of control while doing so.
When you see pro riders cornering while sitting you should remember that a lot of the time they weigh the outside peg (pushing) so much that their butt is off the seat. Think about how your weight is applied to the bike in those two scenarios:
While standing (as well as sitting), you need to move around all the time, to control the Weight Transfer and balance of the bike.
You will move to the front when accelerating hard, going uphill or sit-down cornering.
You will move back when braking hard, going down-hill or when traction becomes problematic (sand, mud, rocks etc).
You move side to side to balance the bike when accelerating or while cornering, change line and bike position etc.
The classic motocross style cornering is sitting down, with the butt as far forward as possible, inside leg strait ahead and out of the way and weight on both the outside peg and the top edge of the seat.
In most cornering situations in the woods, you are not using the leg out - there's enough green things that can't wait to grab your foot. Keep both feet in the pegs and weigh the outside peg.
A good tip here is that you should try to push into that outside peg (and into the tank with the knee) so hard that you lift your butt 1 inch off the seat - this will give you plenty of traction to go fast in these turns. You should practice this position, both while pushing hard and while not and notice the differences for yourself.
It is hard to get one to stand in the beginning. Try to stand as much as possible and avoid sitting down even in tight turns. Look at a lot of videos of trials riders and try to do that in turns rather than sit down - essentially stand strait on the pegs and lean the bike underneath you, into the corner.
It will be slow at first but will get faster as you get used to it. Just keep repeating to yourself while riding the woods: "sitting baaad - standing gooood, sitting baaad standing gooood".
The slow riding exercises from the Basic skills practice section are very good daily practice to help consollidate the standing position.
Great video from Mr Semics:
Here are some supporting videos and articles: