Sticking leg out.

[stevem]

When the Gp guys are sticking their leg out into corners it is actually for a long time ,what are they doing changing gears wise?

[Metal-Man]

they've already changed down gears. They do it to help them balance/counter-balance the bike.

[Mstevo]

they've already changed down gears. They do it to help them balance/counter-balance the bike.

actually they are, blocking other riders ;-), but more it's trying to drag the bike down into the corner.
You take your foot off the foot peg and you'll start to fall off the bike on that side, they are trying to defeat the gyroscopic stand up straight effect of the spinning wheels, hence pulling the bike to that side, it also catches the wind to help do the same thing.
In other words there are a whole lot of reasons, but I think the main one is physiological, "if you think it helps then it does" ;-)