I have a theory that explains the final getsuga tenshou, and canon seems to support it.

Remember the very basis of Ichigo's Bankai? It compressed his spiritual power into a small form, which gave him explosive speed, power, durability, etc. It allowed him to fight at full power.

Well the form we see when he fought Aizen is the true form of his bankai. Remember its said to take at least 10 years of training to control the Bankai. Its why we see Toshiro still training with his, and many opponents have noted that because he is so young he likely doesnt have the control other captains have over their Bankai.

Now if we consider the form Ichigo took to be his true Bankai, that would mean its ability to compress his Reiatsu was so great that he was no longer even emitting spiritual power. In essence - Every. Single. Bit. Of his power was his and his alone. No emissions meant no wasted wasted energy. Imagine if you will a focused flame, there is NO wasted heat that simply disperses in the air, ALL of it focused until a single point.

Thats my theory of his Bankai.