When absorbing energy, it wasn't instant that it would be added to his own. There was a process which took some time.
He could still use the added energy almost instantly, but it wouldn't actually be completely absorbed into his own if he used it this way.
During the times he had used the extraction technique again and again, he had been experimenting with it.
Absorbing energy from those he fought against, if he used it straight away, it helped him fight using Qi for longer periods, but if he wanted to gain strength from it, It was best that he didn't fight and waited for it to be processed through his body and added to his dantian, and now it had finally done it.
All of the extra people he had absorbed and the main disciples' Qi, which was more pure and more refined compared to that of most of those he had absorbed before, was causing him to break into the sixth stage.
