Mainlining builds a deliberately symmetrical plant by topping above the third node and training the two shoots into an even manifold. It costs several weeks of vegetative time, which makes it pointless for autoflowers, and the training comparisons on
https://kushindicaseeds.com weigh it against simpler low stress methods. Short indicas suit it reasonably well. The payoff is a flat canopy of similar sized colas rather than one dominant top.
