What is a "short" jus? Apparently it's a technique that Michel Bras invented but how does it differ from a regular jus?

A short jus is a process of glazing and deglazing, whereas a jus is the meat juices that are produced from the drippings of a roast. Au jus translates as with juice. Jus lié is the meat juices from a roast that have been thickened slightly, with either cornstarch or arrowroot. A short jus is a matter of rendering, hypothetically, duck breast skin, removing the fat, deglazing the pan with duck stock, reducing that stock to a glaze and adding more stock to repeat the process. I would say it's referred to as short because it's a process that is normally done stovetop while the protein is sauteing or resting, rather than waiting for a larger roast to finish.