I struggle to concentrate if I'm not rocking. This isn't just with Qur'aan though, any book I read, or even if I'm listening to a lecture, reading something on the internet or whatever I'm rocking on the sofa while doing it. Not rocking just gets me really uncomfortable. So when I recite Qur'aan I rock then too. Though I am fully aware it looks very strange and thus don't do it in public. Sister Salikah raised a good point which has been on my mind for quite some time in that you should try to stay still. I always think, if I could imagine myself in front of Allah SWT I wouldn't dare to rock while worshipping him, and yet not rocking really knaws away at me. I usually end up thinking surely the important thing is I can concentrate while rocking so that is better (and not against Islam) rather than sitting still and getting frustrated while worshipping. I dont know, it's a strange one. I think I am an extreme example though since I rock in and out of worship.