Ya'll crazy. This chapter was awesome.
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I've found some pretty interesting things by digging through the old bleach chapters:
Hollows attacking shinigamis, their presense masked, gin+aizen show up to save the day:
http://www.mangahere.com/manga/bleach/v15/c130.5/
Look at the chapters name, fusion of a hollow and a shinigami eh?:
http://www.mangahere.com/manga/bleach/v16/c136/
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Also I just had a moment of thoughts so stupid they may be on the right track. Bear with me for a bit here, spoilered part of the fluff to cut down post length: Isshin has used getsuga before, also I'm pretty sure his sword wasn't even in shikai when he used it. Isshin just unleashed his shikai with "burn", leading me towards thinking that his moves shouldn't be something like getsuga.
Ichigo is constantly shown as a fast learner and fast adapter, Gin specifically mentions this about him as it being what makes him scary, and it's mentioned by many other people. Shortly after he becomes a pseudo-shinigami at the beginning of the manga he's able to do stuff like visualizing spirit threads to find shit. Until he fights the menos grande the only way he attacks is with close range slashes. In the mid thirties he meets Ishida and witnesses him using a spirit bow to kill hollows from a range. In chapter 49 against the menos when Ishida is touching Ichigo's initial sword and uses his bow the power flowed through automatically and made his bow wumbo. Ichigo is then attacked by a cero and counters by firing a ranged blast/cut of spiritual power for the first time. Immediately afterwards his power was going out of control and his sword was actually deforming.
When Ichigo finally calls out his Shikai for the first time he unleashes yet another sharp large blast of reiatsu. When he finishes his bankai training he finally becomes able to control getsuga tenshou. It's defined as instantly storing his energy in the blade and then condensing it and releasing it through the tip.
A quincy uses the reishi in the atmosphere and combines it with their spirit power to materialize spirit weapons. A shinigami channels their spirit power into/with a zanpakuto or into pure kidou attacks. Since Ichigo is both quincy and shinigami he is able to channel is own spirit power to manifest/materialize his spirit weapon on his own. I think getsuga is simply an imitation of a quincy attack. Instead of gathering outside energy into the shape of an arrow and firing it he gathers energy from inside himself into his "bow" (zangetsu) and fires it. As Isshin spent so long with Masaki he learned how to perform the same type of imitation.
The final getsuga tenshou is exactly the same as Letzt Stil. He lost all his spirit powers by activating and using it the same way Uryuu Ishida did. When Ichigo was stabbed with a sword full of energy to get his spirit powers back he was stabbed in the same exact spot that Ryuuken Ishida shot Uryuu with an arrow to get him his spirit powers back. (19mm to the right of the heart.)