Yeah, for "I'm not going to lose by undersestimating the Straw Hats" guy, Doflamingo sure likes to undersestimate Straw Hats.
Did he though? From what I understand he seemed to be dissapointed that the trap even actually worked, which means that he overestimated Luffy.
Anyway I really like that so far its been DD against Law and Luffy is just the stranger that wandered into their personal story. Usually the SH's walk in and suddenly those personal histories to the antagonist don't matter as they are there to solve the problem
–- Update From New Post Merge ---
The title is "Palm"… I definitely missed anything in the chapter that even suggest that. So why is it "Palm"?
Doflamingo had them in "the palm of his hand". Seriously he could have ended them right there and then but it seems he's still hestitant about killing Law.
There is an affection there that's been hinted at that stops him from dealing that final blow to him.