Why didn't Hashirama break free from Edo Tensei in the Chuunnin exams? Spoiler Alert: For those who haven't seen episode 366 of Naruto Shippuuden.

As shown in episode 366 of Naruto Shippuuden, Orochimaru was able to restrict the movements of nidaime hokage Tobirama. But he mentions that Hashirama could have undone his binding at any time. 

My question is why didn't Hashirama undo the binding during his reanimation at Chunnin exams?
You will note that Tobirama, in the same scene tells Orochimaru that achieving better fidelity/accuracy/efficacy with edo tensei will be the cause of his downfall, since this time they have been revived with nearly their full power.

This strongly suggests that Orochimaru did not perfect edo tensei at the time of the Chunnin exams and that Hashirama was not at full strength and capacity.

Which explains why he did undo the binding at the time of the Chunnin exams - he couldn't.

You will also note that Hashirama doesn't seem to recall Orochimaru, further suggesting that he was not fully revived at the time, though that could simply be his personality...