When someone is fucking up and their superior notices that they are fucking up, said person is put on notice. This notice, which is essentially a warning, reminds the person that their superior can fire them, punish them, etc. Thus they are "at will" of the superior.
Tina doesn't do shit at work. She just sits around, surfs the web web, and talks to Rob. If she gets caught, she's going to be at will.
Coming from the authority directly above you in the chain of command, this simply allows the individual to choose his/her rate of fire and prefered style, whether open or snipe etc.
To "fire at Will" is the act of giving in to the urge to shoot at a person named Will. This "Anti-Williamism" has been around for centuries. Some philosophers say we are born with the instict to open fire at people named Will. Some philosophers are also on crack. As am I.
I think I hear the police outside my house. Maybe it's because I fire at Will at least once every day. Now where's that crack...