Niara Isley says they regress your age back to when you were taken
"They take you out of your life, at one point in time, and they send you someplace and you do a job there for 20 years for lack of a better term, and then they take you back, they put you in a medbed and they regress your age back to the age it was when you were taken originally, and they put you back into your life, and here's a mind bender, just before you were taken, that way there's no missing time to track."
https://www.youtube.com/watch?v=EP-5tLp5xv8&t=2585s
"It's a matter of regrowing the telomeres in the human body and doing another frequency-based restorative process".