TOPPING
Some marijuana growers will take a pair of clippers to the top of their plant just above the last branch formation during the third or fourth week of vegetative growth. The top is removed by shearing it away at the stem. What happens next is that the main stem splits off in two or more directions, creating a V-shape at the top of your plant. The end result after flowering is two or more top colas instead of one. Now, two top colas instead of one does sound appealing and some growers have even managed to force a plant to grow more than six top colas using this method. Unfortunately this topping method of pruning doesn't always lead to better results.

Depending on the strain and the growing environment, the 'topped' plant may produce two small top colas instead of two big ones. Also, each strain has a threshold for bud production that cannot be improved upon because it is a genetically predetermined factor. On the other hand, some plants when fully grown without topping do not reach their threshold. The strain Blueberry is a good example of this. If you grow Blueberry without topping you won't achieve maximum bud production from that plant, but if you top the Blueberry, you will. Other strains aren't so flexible and the two top colas will simply share the same volume of bud that a single cola would have produced on the same strain.

It's advised that you keep in mind that pruning for yield using the topping method is strain-dependent and experiment carefully with this pruning method. Do this with 2 out of 10 plants in every grow. You'll find in time that during this vegetative prune you will be able to shape your plant. Plants are generally pruned three to four weeks into their vegetative cycle, but can be pruned sooner or later or more than once.

Pruning during flowering is not advised as the plant will be forced to divert its energy from bud production into branch and leaf production. This results in a slower rate of bud growth. For optimal growth finish your pruning well before flowering.

FIM Technique
There is a topping method known as the FIM technique. If you push the leaves apart at the very top of the plant you should see a small bud (not flowering bud but an actually leaf bud). Use a pair of nail clippers to pinch off about 3/4 of the bud. This should result in more than two top colas being developed. In a single FIM clipping you can produce up to eight new top colas.
The origins of this technique are humorous. As the story goes, FIM was discovered accidentally when a grower messed up a topping exercise. FIM stands for: "Fuck I Missed".