Muscle memory is a phenomenon where your body can almost 'remember' previous levels of development. When you try to reach previous highs, it can get there at an accelerated rate.

Here's an example. Let's say you gained 20 lbs of muscle then through other circumstances lost it again. If you returned to the gym to recover that extra weight, you’d get there quicker than you did the first time.

No one really knows the exact mechanisms as to why this is, but it's something that has been observed. If it works, don't knock it!