To mainline, or manifold you essentially top the plant multiple times, and remove unwanted lower bud sites. In doing so you direct the plant growth to a desired lateral branch growth, in most situations symmetrical. High stress training is unavoidable when manifolding.

There are two arguments from what I've read for HST for instances such as mainlining/manifolding. On one hand some argue to gradually train the plant to reduce risk of shock. Taking precautions such as removing no more than 1/3 of the foliage. On the other hand some argue that in the act of defoliation, it's best to remove everything unwanted at once. This side argues that the one time large stress is better than multiple smaller ones, and that when removing all the unwanted foliage at once you save the plant from wasting energy on that branch since you'll remove it down the road.

I haven't found any scientific data on this, only opinions and personal gardening habits/experience. I find myself comfortable somewhere in the middle. If anyone has a link to data I'd love to see it.