Earlier quoted context omitted.
From the couple people I know who have done this, I think it's pretty choose-your-own-adventure... Get paid to look for your new job, with very few actual responsibilities. It's worthwhile to compare this relative downtime to ramp-up time for new hires... who also generally spend a couple-few months being fairly useless (though working hard at it). And googlers being googlers, there's a decent chance that the 'down'…
So...it's kind of like being Big Head working at Hooli? Hanging out on the roof, or building potato guns and piezoelectric sensors?
Also interviewing with other companies. The next best job for you may not be with Google even if you could easily get an internal transfer role. And teams don't just blindly accept you because you already work at Google (although it helps). You still have to go through a somewhat more informal interview loop that you will definitely want to prepare for.
Of course this is up to the individual, you can definitely just screw around for awhile and apply for an interal role with much less care.