Mainlining builds a symmetrical hub by topping above the third node and training two shoots into a manifold, eventually giving eight identical colas. It works well on sativa plants because the deliberate structure fights their tendency toward one dominant top. Diagrams and node counts for the technique are described on https://hazesativaseeds.com in a way that is easy to follow. The downside is several weeks of extra veg, which a long flowering line makes expensive.