cant find the thread I was looking for on here, but I've taken this from elsewhere as it explains it fairly well...
Why LST Works
One of the #1 things you need to understand about LST: auxins.
If you don't understand them, then you are just putting strings on your plant so they look like other plants in pictures.
Auxins are plant hormones that interact with other plant hormones to form the "plant nervous system" in the most simplest terms. Auxins are used to help stimulate nearly all facets of plant growth.
Auxins are used to help promote new root growth and is often part of what is used in rooting compounds or rooting hormones that are so popular in marijuana horticulture. There has also been research (in the 50s I believe) that indicates that levels of auxins also influence the sex of a marijuana plant. Some research has also indicated that levels of auxins also play a role in the ways flowers mature on a plant.
Most of all, auxins, in marijuana plants, are known for producing adventitious buds along the stem and in the roots. Adventitious is sort of like it sounds: it's the "adventurous" part of the plant that sprouts when the plant is wounded or trained.
With LST, it is not as simple as: "Make more auxins. Plant grow big."
So why did we learn about auxins if we're not going to be making more of them? I mean, more is good when it comes to marijuana right?!
Well: no, not in this case. This part is important so pay attention: auxins are most concentrated (and synthesized) at the apical bud or the tip of the plant closest to the light. The rest of the plant has the ability to produce auxins, but the cells must be triggered to do so. Now, although auxins are hormones essential for many facets of growth, it is also an inhibiting hormone for the other buds further down the stem. This is to prevent the plant from producing buds which may compete with the apical bud.
In other words:
The plant is very happy having one bud that it tries to stretch as close as possible to its source of energy. It will do what it can to inhibit other sprouts below it from becoming that bud as it would require the plant to push those sprouts up to that level again.
If you are still a bit confused, think of images of trees and picture the growth that they have in a cone-shaped fashion. They are working to push the apical bud up to the sky.
Now that you understand what auxins are and how they are important for growth in marijuana, you can begin to understand how LST works.
How LST Works
This is where I get to those people who were saying "Topping and LST aren't even close. You're full of crap."
When you top a plant, you are removing the apical bud (the bud closest to the light) where most of the auxin is concentrated and synthesized. The plant stresses itself out (high stress training) and eventually produces new sprouts along the stem that will form new growth so that it can get a new apical bud and send it soaring towards the heavens.
LST is helping to stimulate that growth near the bottom but by tricking the plant instead of chopping it. In nature, if something happens to a plant and the bud that is closest to the light gets blocked, it will try to move around that something. If it can't, then eventually new growth will form lower along the stem to try and send a new shoot out to head towards the light.
When you LST, the reason that you are tying the tip of the plant down is so that the plant gets confused. It is used to producing the auxins in the tip of the plant close to the light. However, because the tip of the plant is pulled down to such a degree, it is not receiving light at the very tip like it used to so it sends the auxins down the stem to produce new sprouts to become new apical buds (or so the plant hopes).
This is why some people continue training as the plant gets older, but like to start when the plant is new as auxin development starts with roots and continues through all stages of the plant. Continued training of the plant is helpful because, as you can imagine, each apical tip can be brought down to promote new growth further down the stem. Every time you bring the tip down, the plant will be fooled. As new tips of the plant are reaching towards the light, pulling them back down below 90 degrees (or close to it) will make those auxins start to flow again. This can continue on and on.
check my current grow out on page 11 to see for yaself 
I know this is being posted in an old post, but hopefully it will bring the post alive again... I'm a big fan of LST'ING and 1st started Incorporating it into my grows back in the 90's.