User blog comment:Nick D Wolfwood/Questions questions questions!! Kurosakigate.../@comment-2129921-20100624181352

Don't know if this is relavent but i just blew my own mind, if i remember correctly Isshin used GT with such fierocity that Aizen himself said he would have succumbed had he not gained mastery over the hogyoku, Isshin's zan was sealed when he used this technique. So either Isshin is an absolute monster in terms of spiritual pressure or he released off screen.