Actually, the bind MrPlayskool gave will turn on Super Speed and cause your character to start hopping up and down endlessly.
/+up = when bound to a key, jump (or fly up) as long as the key is pressed
/++up = jump, and keep going up even after the key is released, and when you land, do it again.
Much like the default bindings of W (+forward) and R (++forward).
I'm not 100% sure what you mean by "Speed on Demand", but I'm assuming you want Sprint or SS to turn on when you press a movement key, and turn off when you release it. If that's it, try something like this:
/bind t "+forward$$powexecname Sprint"
This only works if Sprint is off to begin with, or else it will stay on. Also if you want it for each direction, you will have to bind separate keys for +forward, +backward, +left and +right.
Hope that helps!
