In a lot of our Katachi waza, Uchidachi pushes Shidachi backwards then cuts.
To be able to push your opponent, your kissaki needs to be in the center. When you push him you shuffle your front foot to enter into his space - your kissaki aimed at his face or chest. You keep pressing by taking a full step ahead with your back foot while raising your sword. When your opponent steps back, you cut him.
It is important to cut him WHILE he is stepping back - while his backing foot is still in the air. If your cut comes when his foot has already landed, it is too late. He could - and should - cut you.
If you chamber your sword behind your head, you give your opponent the opportunity to cut while it is behind you. You created an opportunity for you when you forced him backward, and you just lost it by chambering behind your head. Not very bright, you deserve to die !
Of course if both of you chamber behind your head, this does not apply, maybe you both deserve to die !
I suppose this is why in Itto Ryu we are told not to chamber our sword past our head, and why in Kendo the Men cut keeps the sword moving forward.
Practice this : Start at one end of the dojo and push your partner - ask him to cut you if he can - if you chamber behind your head, he should seize the opportunity. If you only chamber above your head, he should have no opening.
BTW, this also works in Karate. It is very difficult to launch a successful move while retreating. Unless you are baiting the opponent - but this is a different story !