As grafix has pointed out its when you have injected the needle in and then pull back on the plunger to make sure you are not shooting into a vein. If you pull back and blood rushes into the syringe, you are in a vein and need to reinject elsewhere.
Softly draw back on the plunger for 1-3 seconds to see if blood appears. If it does, this means you are sitting in a blood vessel and this is not good. Simply remove the needle, discard the needle and solution and start over again.
Upon no blood appearing after aspiration, inject solution very slowly (1ml/min). The slower the injection, the less the trauma to the muscle tissue and the less the discomfort you will feel.