I am new to boating and have just purchased my first boat. I purchased a '63 Mark Twain with a 75 hp Johnson outboard. I have taken the boat out on the lake a couple of times already and each time it is a new adventure (they should all be posted on stupid boating tricks). But anyway, On the throttle arm (or whatever you call that thing) there is a forward and reverse lever, a choke lever, and a small twist lever that says "throttle pressure." I am not real sure that pressure is actually what it says, but that is the best that I can make that out to be. Can anyone please tell me what that actually is and what is its function? 
