“Main-lining” is the process of training your cannabis plant (utilizing Topping, LST, and Lollipopping) to form a manifold of branches where each branch has a direct line to the main stem. It is a great technique to control the growth of both indoor and outdoor plants. Most importantly, main-lining is a great way to increase yields by allowing for a fuller and more even canopy. This technique was coined and introduced by Rollitup.org forum user Nugbuckets.
Naturally grown (or traditionally grown) cannabis will form branches of various lengths at various points along the main stem leading to uneven branch and bud sizes. In naturally grown plants, the Apex Bud (also called the Terminal Bud) will always be the biggest and strongest due to it having a direct line to the main stem and it being the highest point of the canopy (closest to the light). This phenomenon is called Apical Dominance (sending most energy through the main stem to the top of the plant), which is what we are trying to manipulate or avoid with the main-line technique.
In a sense, main-lining allows every cola to grow as if it were the apex bud. By topping the main stem and training the branches, we can transfer the energy from the apex bud to the lateral buds. Since each branch now has a direct line to the main stem and equal access to light, they will receive proportionate energy and grow uniformly.