Zen is all about acceptance. The reason why monks sit in that position is because it is the ultimate position to for acceptance. By sitting like this you stop using your feet and hands, the things that most make us human.

More than half of the bones in your body are in your hands and your feet. This is because humans have evolved to use them almost exclusively. Sitting in Zazen position it to essentially stop being human.

In addition, in the Zazen position, you still allow your vision and hearing to bring information in. Your eyes are half closed and you don't focus on one particular thing, rather your vision blurs and you take in everything. In the same vein, you do use your ears to listen, rather to hear.

Moreover, when you sit down and try to do nothing, many thoughts come into your brain. This is natural and is what it means to be human. The difference here is what you do with these thoughts. If you dwell on them you need to change your course of action. The idea is to not dwell on the thoughts and just let them keep flowing through you. This is what it means to release the thoughts.

My feeling on this is that in this case acceptance and release are exactly the same thing. To accept means to release. The sooner you can accept things, the sooner you can release them, and the sooner you are free.