The functionality of it wasn't really spelled out during the ALPHA, and lets face it, not much worked back then. But as I recall, it seemed to me that you could either attack right away for an accuracy bonus, or wait until it was completely charged and get a power bonus instead. (ALPHA was so buggy, it was hard to even get a baseline idea of how things were supposed to work so we could evaluate them)
It was a rough concept then, and it shouldn't have existed for normal attacks, but if it were to be revamped and reintroduced with certain TP attacks, I think it would really set FFXIV's battle system apart.