No, you don't understand what the rule is. It's not an extra play, it's just a removal of the nonsensical 'advantage' situation which is a grey area and penalises teams that want to attack. There were a bunch of whining idiots over here too when it was first introduced who just saw the words 'free play' and started moaning without even knowing what it was, but it now has universal approval and is probably the best rule change that the sport over here has made in the last 10 years.
I've seen it in practice and still think it's a stupid f##king rule.
And from what I've seen of it in practice, free play is a perfectly acceptable name for it because that is pretty much what it is.
It's basically a free ticket to do whatever you want with the ball without any consequences provided you're not tackled for one play simply because the opposition made a mistake, I've seen teams knock-on only for the opposition to pick up the ball and get a "free play" where they run the ball 50 meters clearly gaining an advantage because of the oppositions mistake to then knock the ball on themselves and be marched back down to where the original knock-on happened and receive the ball back, and I'm sorry but that's horse sh!t, why for that one play should their be no consequences for making mistakes that there're normally consequences for.
There's nothing nonsensical or grey about the advantage rule at all, in fact it's pretty bloody simple, if the other team makes a mistake it's a hand over, unless your team gains an advantage from that mistake by crossing the advantage line, in which case it's play on as the other team has already been punished enough for their mistake.
Sure I can see ways that the advantage rule could be improved, but the "free play" rule is way to lenient in the other direction and not the answer to the problem, if there is even a problem which I think is also arguable considering that I have never in my life heard anyone complain about the advantage rule until the RFL introduced the free play rule.