The Avatar State was an asset for that additional spiritual strength he needed (man how many times have I said 'spirit' in one post?). He simply had to feel stronger spiritually, and that would take over as soon as he had to take away Ozai's bending. The Lion Turtle passed down the form and knowledge to him through spirit, and his spiritual connection did the rest. I don't believe Aang knew exactly what he was doing there physically. I think it was probably a bit of both, if that makes sense. The main reason why I initially didnРІР‚в„ўt assume Aang used the AS here is because - even though thereРІР‚в„ўs plenty of consistency evidence with other instances of energybending - it is heavily implied that his first time entering the AS at will is when he extinguished the flames in Wulong Forest. If you donРІР‚в„ўt mind:Р”Вo you think that the AS occurred in that moment by his own will or rather that it was still РІР‚Сњa defense mechanismРІР‚Сљ?

As a final note, the requirement for the Avatar State also supports that the Avatar is the only bender who can use this skill, and it is not something to be learned by someone else, as the plot point of the first book of the second series with Amon's abilities Thanks for the breakdown. The arrow tattoo glowing is more than likely connected to a monk Avatar's higher spiritual connection, so I would not be too surprised if it can happen outside the Avatar State as well, but the fact that the sequence started off with them not glowing, and then they did at that moment, suggests that he entered the Avatar State when he tried to actually bend Ozai's chi, instead of simply realising his own. This indicates that the actual power to bend the energy within another and one-self requires entering the Avatar State, in my opinion. You will also notice that his arrow tattoos glow separately from the rest of his body, much like their eyes and mouths did, during the energy bending struggle:, which, if he weren't using the Avatar State - when that phenomenon occurs - should not be happening ( as we see earlier, his tattoos weren't glowing until that point in time when he fully entered a spiritual struggle with Ozai to take away his bending). That said, we can also see it for a split second in this very scene as the aura / light of his energy fades away, Aang's eyes and arrow tattoos are still glowing. The last time we see the technique being used, against Kuvira's spirit cannon, Korra once again entered the Avatar State to energybend.

I will also add that when Aang gave Korra her bending back, and when Korra gave Lin her bending back, the use of the Avatar State was equally apparent. As already mentioned, we could see him use it when he took Yakone's bending away, which adds to consistency.

They should make a new avatar series when aang was older