The secret was always Zoro's bandana... It'll be a legendary icon, as great as Luffy's Strawhat.
On a more serious note: This was a nice chapter showing Zoro's growing understanding of Conqueror's Haki, and in a way it works out. After all, when Mihawk trained Zoro; Zoro didn't have anyone to exert his will over and/or tame like how Luffy had in the island where he trained in 2 years when it was a matter of survival. Sure there were the monkeys, but Zoro had defeated them, and like Mihawk, Zoro doesn't make it a habit of constantly beating people whom he already defeated.
So Zoro developing his conqueror's haki through this method, of using Sommers as a training dummy is suitable, as not only is Sommers immortal, but also augmented with Imu's omen.
All in all, great chapter, and bye Sommers it was nice knowing you... Well not really, but you were entertaining when you were getting thrashed.