The difference between HST and LST is in the name. HST(high stress training) involves cutting and breaking of the plant to achieve some result (typically slowing verticle growth or creating multiple stems and/or colas).
This activity causes considerable stress to the plant and could concievably injure the plant to the point of stunting or even killing it. The key to HST is to do it when the plant is at its healthiest and in the strongest mode of growth so that it can handle the stress and heal itself. It also requires considerable knowledge and experience that can only be achieved by injuring and/or killing a given number of plants with this method
I have literally
broken branches and bent them horizontal and came back a few days later expecting them to be dead, only to find that they healed themselves and had straightened back halfway upright. Young vegging MJ, when healthy, is very resilient.
Some people are not comfortable doing HST or just don't think it is necessary to achieve results when LST(low stress training) is a viable alternative. LST involves manipulating the plant in any way that does not do injury to the flesh of the plant.
You can tie down a plant from its first week to its last without causing any stress. Even an older plant can be tied and bent as far as possible without breaking, and then adjusted more every few days until the plant is bent as far as desired.
More often when a plant has grown too tall and stiff to bend, the upper portion and/or the longer side branches can be tied to allow more light penetration into the depths of the plant to encourage lower bud maturation.
In my opinion and experience, the best use of LST is when a plant is at 3-4 weeks old, it is bent over to encourage it to grow on a low diagnal. This allows and encourages all of the side branches to to grow upward, which when taller than the meristem, will be given the hormones that promote main cola growth and development.
There is another very good method that goes with this called "ScrOG". That is a "screen of green". By using a screen(with 1-3" holes) low over the growing plant, it is encouraged to grow horizontally beneath the screen, which allows equal light penetration to all of the plant. This also encourages the side branches and tertiary growth to reach for the light and become main colas.
Both LST and ScrOG are usefull methods for keeping a plant growing low and for maximizing the bud production of a single plant. I hope this gives you some perspective on these methods.