don't know if there are any actual scientific studies, but the term is loosely used to describe how when after you've worked up to a certain level of muscle mass and/or training level, then lose some or most after a layoff, it is much easier/quicker to gain back the mass and/or training level than it was to build it the first time.
the only "study" i can think of is the casey viator "colorado experiment" conducted by the guy who invented nautilus equipment. Viator regained ~~ 50 lbs in a month.
he spun it as an endorsement of how great his equipment was, but it was really a demonstration of muscle memory and juice.