The stress of beating the tree induces the tree to produce induces Ethylene,resulting in a larger yield the following season. Here, the stress is created by bending a branch up or down, but the result is the same.
http://link.springer.com/article/10.1023/A:1005948918382
“Mechanically induced stress (MIS) was imposed on apple shoots by bending horizontal shoots upward and vertical shoots downward and its effect on internal ethylene evolution, polar auxin transport and cytokinin levels studied. Induction of stress caused a significant rise in internal ethylene production in the tissues under stress, decreased polar auxin transport and cytokinin levels and increased the percentage of floral buds in the shoots.... more at link”
I’m compelled to point out that “auxin” is a compound created in a plant’s roots and transported up into the plant, and is the thing that stimulates new growth at the growing tips of the branches/limbs.
Auxin is alto present in fast growing annuals like cannabis, but I would not know anything about that.