Board Thread:Theories and Speculation/@comment-30168130-20170710055942/@comment-734582-20170710083423

As explained by Jiraiya and later Kakashi, there are 3-4 steps for the Rasengan/Wind style ransengan.

Chakra concentration/rotation (Balloon) Power/chaotic motion (rubber ball) Control/compaction (balloon kept whole/ tree impact)

Wind Style adds a 4th: Nature transform. (in the case of wind: grinding chakra against itself)

Boruto however added the release by accident. He did do step 1 and 2 (see: movie/manga) but in stead of step 3 (where he expands it to full size and controls it fully), he apparently did Wind Release. As i noted before, there is a considerable similarity between step 2 and 4 (for wind release). Imagine for a moment a circular motion, and then a second motion exactly opposite to that. At the point of contact between the two, you have the condition for wind release (splitting chakra and grinding it against itself). Interestingly for a rapidly spinning sphere, this would manifest as a plane along which Wind Release is the most intense, which is where the blades of the Rasenshuriken could come form.

In the end, it wouldn't take much more effort to go to a regular Rasengan, since Boruto could relax his control over the rotation (thus breaking condition 4) and in stead focus on his control over the power (thus meeting step 3).

Naruto's style of Wind Release Rasengan wasn't the most efficient (he made a rasengan that he then added wind release to, which was extremely hard to control). From these steps the logical route is: 1: Concentrate chakra and nature convert it. 2: rotate the chakra violently. This sets up the two essential components of Rasenshuriken: nature and shape control. 3: Increase the power of the technique (step 2) and the shape/control of the technique (step 3) iteratively.

The result from this would be exactly what we saw Boruto create, namely a mini wind rasengan. To make a full-fledged one he would have to train it's use and scope, but notably the most elementary part (heh) of the jutsu is already in place.