You remain employed during this time off, there is no re-hiring involved.
I've seen it happen, people take extended leave of absence on FAANG, to pursue some non profit stuff or simply to prevent burnout.
During those, you're still covered by healthcare plan, but everything else freezes: let's say you take 2 months off. Then your RSU that was supposed to vest next month is now vesting in 3 months. Your vacation days don't accrue during those 2 months, etc. Obviously, since you're not getting any salary, there's no contribution to 401k happening during those 2 months.