In reply to Peter Walker:
The impression I got from the book was that Emmett started downclimbing to reduce the distance he would fall (and therefore would be slightly less likely to hit the ledge) but was soon off - it was hardly a casual jump! I think Dawes envisaged the belayer jumping off but Emmett's attempt was the first time the theory was tested.
Classic quote! From memory:
Emmett: "I don't think I'm up for it man"
Gresham: "What?! You've GOT to be up for it!"