Insteadof threading the freewheel onto the hub, the engagement device is built directly into the hub. Then you just screw on different metal rings with teeth to act as freewheels. You can run low ratios (25-9) and there's instant engagement
it is possible to make one that has a very small amount of movement between engagements to make it feel almost instant, but in order to do this you have to have more engagements in the hub. this means smaller engagements and smaller pawls, which also means weaker parts.
an in between needs to be reached between size/weight/engagements and strength.
not many nore improvements can be made now using the current systems in hubs.
any major improvements from now on will involve radical ways of engageing a hub. Chris King use a patented engaging mechanism, but it is still to expensive for the vast majority of the world.