topping will cause the energy/growth that would normally have occurred at the top/tip to be redirected to the lower parts of the plant because the "top" is gone and will not ever receive the energy/growth it would have.
What will happen is the plant will widen even more because the energy will go to the branches and stems that are below the top you just cut and it will get bushier, typically.
I believe most people "Top" with the intention of an increased yield by increasing the number of buds. For my scrog method I believe topping will be used whilst my clones are vegging in veg area, but not once they are in the box because it slows growth due to shock. "Topping" the main stem will cause the main stem to branch into 2 stems, doubling it. THis will also give you two main colas instead of one cola because the plant divides into two. You can do this over and over, but there are consequences...
Think of it like this. If you have a water hose and you turn it on full blast, the water will shoot from the end with force. If you split it with a "Y" and then split it again with a "Y" (giving you 4 hose ends) the water will no longer shoot but trickle. It's sort of the same thing. WHen you top, you divide/split/double the main stalk, creating more main colas and if you topped 2 times (like the hose), you will have 4 main stalks/colas/buds but the energy that would have made on huge nug will now be directed between the many. THere has to be a way to balance out how the plant justifies the energy it creates, but I don't know that yet.
Topping also forces the plant to grow more secondary shoots(tertiary shoots I think they are called?) which means longer branches and more buds, but the energy is divided amongst them.
THat is as far I can go with this FACT, I need to read more too... lol
Go read and the tell me...
P.S. I just topped my mothers, you can see the results in a few days as the main stalk doubles....