Indoors pruning should take place between 10 and 15 days before you force flowering. If you are growing outdoors, then you should start pruning a few weeks before you expect your plant to start flowering. Tall plants can waste a lot of nutrients and energy supplying water throughout a long stem. Instead of your plant wasting so much energy growing a long flimsy stalk, it could have put more energy into growing you a big fat bud. Don’t start just whacking off leaves and branches!