himespau 

Basically, as I understand it, it's a rough (at least when determined by algorithms), measure of your capacity (in kJ) above your lactate clearance threshold. Or how much energy stores you've got at any given time to go above your FTP. The more time you spend above FTP the more it drops. The higher you are above FTP, the faster it drops. Hang in the draft for a while and it builds back up again (the lower you go below FTP the faster it builds back up). The estimators I've used for it don't seem to do a great job at factoring in long term fatigue (i.e. most of them seem to go back up at the same pace no matter whether it's your 1st or your final effort and always go back to the initial starting value if you recover long enough), so it looks like you could still hit the same power again, when, at least for me, after I've taken it down to 0 (or below) the 2nd or the 3rd time, I can't get it down anywhere near that low again. And yes, I know that means I need to do more intervals.

Last week, I'd done a bunch of 1 off efforts of varying lengths and just about every time I was going well below 0 (-2 to -8.7), and was wondering if an accurate estimate would have a hard stop at 0 or if the metric was based on having already depleted your creatine phosphate stores or whatever and it would be expected that a single effort would be able to get below 0. Then I noticed that after a week of those efforts, my W*Bal value had changed and I needed to enter a new one into my computer. That done, only one of the efforts would have been below 0.