Imagine I have a box (A) and ssh access to it and want to run a programm and forward its x11 output to xMing. That works just fine with putty.
Now things get more complicated. There is another box (B) that I don't have direct ssh access to but I can ssh from box A.
So I ssh to box A with putty and from that box A ssh to box B. How do I run my programm on box B and get X11 forwarded to me?