I guess that if "slowing down" and "slowing up" can mean the same thing, then "going on" and "going off" can do likewise.

The way prepositions are used in coined phrases sometimes seems to have little to do with their literal meanings. We all know the "up" has something to do with height, but when someone asks you to "wait up", they don't literally mean you should climb a ladder and wait at the top rung.

And as far as the "pound sign" thing goes, I'll never get used to it. To me, "#" is the "number sign". I don't think I've ever used it to indicate a pound. It's too easy to use the "lb." abbreviation. When some automated call system tells me to press the "pound sign" I have to stop and think which one it is. Same with the "star" button. It's not a %#$@& star, it's an asterisk!