2 speed rearends

I can't say I wouldn't mind the convenience of one of those, but I think I'd almost rather just get a quick change rear end and just swap the gears when I get to the track. You wouldn't even need extra gears, just drive there with a big gear on the drive side and small one on the driven and switch them around at the track. I wish I could say what exactly I'm talking about. I've seen them on TV before and they seem pretty cool, not sure what kind of power they could handle though.