It means that instead of pulling back on the bowstring the archer would push outward with his left hand, keeping the bowstring in its drawn position. By pushing the actual body of the bow forward with his body and not bothering with pulling back the draw you can partially compensate for the rather absurd draw weight of a longbow. I don't know whether this model is accepted or not because I'm no medievalist, but it certainly seems plausible and helps account both for the ability of English bowmen to fire those things and helps explain one of the many reasons why it would have taken a lifetime of training to learn and keep yourself in practice. As I believe /u/Rittermeister says, the longbow is less like a big bow and more like a human ballista