[He's keeping his hand in place until she's done cutting and once she is, he's ripping it apart to get his hand free. Flexing and curling it makes the joints pop every so often. When he's done with that, he studies his other hand, looking at the straps calmly.]

Let me see if I can do it on my own before we resort using the knife on it.

[Since Aizen was locked up in a place called nothingness, he didn't have much to do beyond stare at his hands and straps and imagine himself tearing out of them so that he could destroy those who imprisoned him. Not surprisingly, Aizen uses those hours of staring to his advantage. Manipulating the straps in seemingly random ways, he's able to free his hand without anymore damage to either Misha or Aizen due to mishandling of a knife.]

...I don't think it would be for the best if you used that knife to remove the straps around my head.