It looks like a more robust illustration of borrowing. Rather than borrowing the ten onto the ones digit, he pulls it to the side and subtracts off the seven before bringing that result back. Orperhqps you could say it is associative property or such... Like:
23 - 7
(10 + 13) - 7
(10 - 7) + 13
3 + 13
16

It may represent a good handle on what they call algebraic thinking.