big loop: dog puts head trough it.
end of leash is attached to wheelchair.
to attach his collar + leash: first make sure that the grip at the end of the leash is fixed to the wheelchair. the dog has to walk forward so the knot will shift and make the loop smaller until it fits.
to detach : attach the small loop you see on the first picture to a hook on the wheelchair (f.e.) tha he has to walk backwards to make the loop around his neck bigger to escape.