When you use enable ICS (Internet Connection Sharing) under Windows w/ your laptop, that's exactly what you're doing. You're letting your PC act as a wireless client bridge by sharing your wireless connection to the wireless router w/ wired devices on its ethernet port.
Thank you very much for your expanation and now I think the following that I want to do is possible:
I have a VDSL2 router w/ wifi-bg and therefore in would like to use wifi-n like this:
do you think this will work?
Yes, it will work. You're essentially turning the wireless-AP-n into a wireless repeater. Normally you would connect the wired side of the bridge to the repeater. But in this case, you’re being smart by connecting the wired side to the primary router so you're able to use wireless N instead of being limited by wireless G. But beyond that, it’s six of one, half a dozen of the other.