stretch training?

what exactly is stretch training and when can i start? is it a good thing or not? what will happen? thank for the help guys
 
i'm going to be using UB's topping technique on my first grow. At the moment they are in the seedling stage - about 1 week old. Just waiting for the 5-6 set of true leaves and then they're facing the CHOP!!! Hoping to get 16 main colas off the 4 plants.
 
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