You can categorize leaves by where they grow on the plant. There are leaves that grow at nodes of the central stem, those directly attached at nodes of secondary stems, those whose petioles attach to the nodes of tertiary stems, and so on. The primary leaves are your first big fan leaves and the first to turn yellow and drop off (these are also the big leaves sticking out of your top cola). These different categories of leaves appear in succession.
I've grown a lot of plants where I trained them down, tightly wound around the rim of a 1gal container. This packs leaves together and though many fan leaves can be tucked, some have to be removed. This is done at the start of 12/12 lighting. I've experimented with cutting as few as possible, and also trimmed almost all the primary leaves which really open the plants up again to air and light. I can't make any certain statement about ultimate yields here, but I've been surprised at how quick the defoliated plants bounce back, producing a full canopy of green again. I've wondered if these are stages in the plant's growth cycle, and if removing these primary leaves before their natural senescence, allows the plant to step out of the vegetative stage, bumping the plant up into flowering mode a bit ahead of schedule. If so, this could explain the claims of higher yields.