User blog comment:Xilinoc/Ch. 571 (Yachiru's gonna wreck it)/@comment-10554972-20140305200621/@comment-10554972-20140305211328

I think it probably depends on the Zanpakuto. If Isshin did use Getsuga Tensho without releasing his Zanpakuto, then I would say that it would likely be weaker, but I think it's different with Rose and Yachiru. The technique Rose used with his sealed Zanpakuto was different than any of the other ones he was shown using in his Shikai. Now I know that he likely has more Shikai techniques that we haven't seen yet, but his sealed technique is probably unique to his Zanpakuto's sealed form. Sort of like how his Shikai is a weaker form of his Bankai, his sealed Zanpakuto might be a weaker form of his Shikai, all of which use different (music based) techniques. Yachiru's is probably similar, in that the technique her sealed Zanpakuto uses is different than the one her Shikai uses, but is similar in its makeup. Because two kids actually manifest, each having a sword of their own, the attack would be stronger since it's actually three attacks, whereas her sealed Zanpakuto just has a margin of error that allows her to cut things that just barely dodged her attack. It's similar, but you can see a difference.

Now back to Isshin. If we assume that sealed < Shikai < Bankai (what I mentioned with Rose), then it might make sense that he can use Getsuga Tensho with his sealed Zanpakuto. Ichigo also uses that technique, which means there's a similarity between Engetsu and Zangetsu, which allows us to draw some conclusions about Engetsu based on Zangetsu. Because Ichigo uses a stronger Getsuga Tensho with his Bankai, Engetsu might be the same (it was also stated that Isshin trained to use the Final Getsuga Tensho, which is probably a Bankai technique). With Rose, his Shikai and Bankai use different techniques, with music being the tying factor, so it makes sense for his sealed Zanpakuto to use different techniques as well. Engetsu (likely) uses at least this one technique in both Shikai and Bankai, so it would make sense that his sealed Zanpakuto would use this same technique instead of a different one, but weaker. Does this make any sense?