You're welcome.
Well is just a pattern that you'll see quite a bit here in the forums and some bits I gathered throughout my time here.
First unless is necessary I always create Timeline instances paused, because otherwise the time is already running and the playhead is moving forward. Granted, normal JS execution to add the instances might take only a few milliseconds but in some cases that could cause some unexpected behaviour. Now I'll jump forward to this:
tl.reversed( !tl.reversed() );
Reversed is a getter/setter of the reverse status of a GSAP instance. By default all GSAP instance are created and going forward, therefore by default reversed() returns false. Since I'm using reversed() as a setter (by passing the opposite value) I need that the reverse status of the timeline to be true. Now since I created the timeline paused the playhead won't go anywhere unless I change that, hence I add a reverse() call at the end of the timeline. Now the timeline is paused, so I'm telling the timeline: "Go from being paused to play backward", but since the timeline is at zero seconds and negative time doesn't exists (although @GreenSock and/or @OSUblake might be working on that ) the timeline stays put at zero seconds and the reversed() value is true. Then on the first click (or any other event that triggers it) I toggle the reversed() value to false, which tells the timeline to go forward and so forth.
Basically is just a way of doing it without the conditional block in it, which saves a tiny amount of time and a couple of lines of code.
Hopefully this makes things more clear about it.
Happy Tweening!!!