Board Thread:Naruto Discussions/@comment-25977287-20150704114842/@comment-681696-20150705003551

Madez wrote:

False.

We saw Minato throw the kunai at madaras feet before entering sage mode, which means Minato moved. If you move even a little while gathering nature energy, then entering sage mode will fail. As Fukasaku said, it's physically impossible to move whole gathering nature energy.

So obviously Minato entered sage mode the very instant after he threw that kunai. There was is no other explanation. His eyes only changed after that kunai was thrown,meaning sage mode was activated right ere and then.

I haven't seen the anime episode yet, I was basing my theory on the manga.