I am not sure myself but it my understanding that it happens pretty much from top to bottom. I think it is really a function of light. Those trichs that are on the top of the plant get the most light for the longest periods, and the further down and into the canopy you go the less mature the trichs are as they don't get as much light.
That is one of the reasons for doing some of the training techniques that we do, to get that all important potent light down to the lower buds for better development. I have been trying to get my partner to understand that point. because of that, if you grow with SOG then the top of the canopy is the focus of your and the light energy for production.
Whereas if you grow with a standard method of allowing the whole plant to produce bud, then you need to do some form of LST in order to open the plant up to allow light to get lower into the plants. The more you want to develop individual plants, the fewer of them you have to start with and allow longer veg time so that the plant can be made to open up and spread out.
The real keys to trich production are: good nutrition and maximum light and air levels.
