It's your mind.
With this gift, you are able to practice your piece without even touching the instrument. Your hands, your voice your moment can be practice any time you choose.
A perfect example of this is when you are just doing something that doesn't require a lot of thought and a piece, song or something musical just pops in your head.
Recently I watched Frozen 2 for probably the 3 or even the 4th time. The composers
of the songs really get it when it comes to something memorable. Mozart is another one. His melodic lines are so good!! I was heading off to bed and after watching the film and the song "Into the Unknown" was repeated over and over and over again until it finally stopped and I was able to go to sleep.
This is the power of your mind.
Enjoy it as you are studying an incredible piece. It truly is a gift.