Well, Here is my take on it ...
We all have read that the Juubi is all Nature chakara, Thus it should be easy to assume that each of the Bijuu are also made of Nature chakara, just not as much as the whole... For example: you have an Orange, you peel it and break it into slices .. each piece is still an orange... just not as Orangy as the whole
Here I want you to remember what Son Goku (4 tails) said when he gave Naruto his gift: "Put your fist with mine, there will be a day when you may need it" (summarized not exact words)
Thus giving Naruto a piece of his Chakara .. as well as the Bijuu ..
So I think that Naruto finally tapped into the chakaras given to him, thus allowing him to jump straight into both modes..
Another option is that Naruto still has a clone out there somewhere gathering Nature chakara, but doubtful...
More importantly though, and I'm quite surprised not to have seen a thread about it ..
Jubito is vulnerable to Sage Chakara, So Where Are Ma and Pa? as they are the master toad sages ..
and will Kurama finally allow a fusion? allowing Naruto to stay in SBM indefinately?