you can super crop around 2 weeks into veg or when you hit about 2-3 nodes. be gentle when you first start super cropping (if you do so after only the 2 node as appeared) so you dont stress the plant too much. the bigger it gets the more aggressive you can be with your super cropping.
when my plant reaches about 12in tall i super crop the **** outta the plant...like every branch...but thats slowly building up over time...just start with one branch at a time in the beginning.
immediately after the super crop that branch will lay limp possibly or look sad (this is normal)...after 1 day usually they perk up and there will be a little knot where you pinched it.
this is good for two reasons: encourages growth and helps support the big heavy buds during flowering...look at is as weight lifting. the more weights your lift the stronger you get. same idea. and instead of building muscles you plant will have an explosion of growth near the pinched site.
if you aren't careful your branch may look like its about to fall off..you can correct that by using a piece of paper and tape or a small flexible piece of cardboard and tape.
leave it for a week then remove it and it should be good as new