Like this:
host1$ ssh -N -R 8822:localhost:22 remote.host.com
The optional -N says "don't execute a command" (helpful to prevent accidents caused by leaving remote shells laying around.)
Now from remote, you can SSH to host1 like this: (The remote port 8822 forwards to host1, but only on the loopback interface.)
remote$ ssh -p 8822 localhost
For extra credit, you can export the forwarding to the whole world, allowing anyone get to host1 by hitting remote's port 8822. (Note the extra initial colon)
host1$ ssh -N -R :8822:localhost:22 remote.host.com