I've recently unlocked the soda popper for the scout class. I love the weapon and use it as my go-to scattergun. In the weapon description it mentions that when hype has been built up, pressing alt+fire will activate a special ability. However, when I press alt+fire with a full hype bar, the weapon fires and nothing else happens. If I right click, the weapon glows purple with no other effect (except I think it might be giving me critical hits from behind; I'm not sure though). Why is nothing happening when I press alt+fire and what is happening when I right click?
First of all, when the weapon description says 'alt fire', they mean alternate fire (right-click by default), not alt and fire at the same time. Many weapons have alternate fire methods, including the pyro's airblast for his flamethrower, and the medic's ubercharge when his uber meter is full.
The weapon discription for the soda popper is somewhat incomplete, due to the unusual nature and history of the Soda Popper.
What it does now is provides 5 additional mid-air jumps (on top of the extra jump Scouts already have, plus any additional jumps granted by other items) while this ability is active. Simply right-click when the meter is full, and you will be granted these additional mid-air jumps.
In this past this weapon would automatically activate, and provide mini-crits. This is likely why the weapon today has an inaccurate description.