Super Cropping, topping and fimming are all ways of pruning your marijuana plant. Here is a quick run-down of each of them.
The basic idea behind super cropping is that if you stop the plant from realizing that it has a top, it will grow outwards instead of upwards thus forming multiple bud sites and colas. While this technique is extremely effective in many cases, some plants dislike it so it's important to do a test run of a few branches before tackling the whole plant, even if every single plant of that strain you've ever grown has been absolutely fine with it. You'll want the plant to be around 3-4 weeks old when you start (i.e. past the seedling stage but still young). Pick your target point on a branch (usually around the 1st and 3rd node sets) and squeeze/pinch every branch which crosses that imaginary horizontal line so that they bend but don't break. The aim is to force the plant to divert nutrients to its recovery (thereby stimulating growth in the lower part of the plant) rather than to cut it back completely. Pinching the stems this way may seem damaging but as long as the branches do not sheer all the way through they will be fine.
Topping involves actually cutting back a plant on the main stem, with the aim of
making it regrow more vigorously than before. It's a very stressful operation so plants need to be well past the seedling stage to be able to survive it. Giving your plant the chance to grow to six nodes and then cutting it back down to a minimum of three may seem like a time drain, but it's worth remembering that heavy-handed and/or premature topping can damage a plant so badly that it never recovers. As an absolute minimum a plant must have at least 3 nodes for topping to be considered. Done properly, topping will cause the top of the plant to split into two lead colas. This increases the yield and encourages the formation of more main colas, which can improve the yield even more.
FIMing (FIM = Fuck I missed) works on the same principle as topping but instead of cutting the plant down, growers simply pinch the stem to inhibit growth. On the one hand, this means that FIMing leads to taller plants, which are less than ideal in some growing environments. On the other hand it means that plants are much less stressed and therefore recover more quickly.
NB: all of these techniques need to be used when plants are still in the vegetative state. Attempting any of these techniques when the plant is flowering will seriously damage it.