Board Thread:Questions and Answers/@comment-9826910-20130618064245/@comment-11051098-20130622104853

No problem. It is the most logic explanation, as I see it. A great power-up for a limited time to greatly increase his power and be able to defeat Aizen.

But basically, Aizen fell because of his own arrogance. He didn't realize that Ichigo was stronger than him until Ichigo told him he was the one who demolished the mountains around them when their blades clashed. He even said, when his blade disintegrated, that the Hôgyoku had determined that he no longer needed a blade, implying that he had reached unknown heights of power. But shortly after, he was discarded by the Hôgyoku.