Greatest thing ever.
It enables you to change your loadout without having to open up your backpack.
Basically, after you bind it, (It's in the keyboard second of your settings) while you're in-game, if you hold down the bound key a little menu will open up.
Now, I have a custom HUD so there's a good chance the stock quickswitch menu looks a little different.
You can scroll up and down on the list and you click on whatever weapon you want to equip.
If you have your melee weapon out, it only lists melee weapons for that class.
While in-game if you look at someone and hit the key inspect is bound to, it will tell you the weapon the person has out.
While if you hit the key while not looking at anyone the window will close. This will also show items that have been name tagged.