Briefly, "backloading" is the practice of drawing up drugs or, more specifically, a drug solution into the syringe of an intravenous drug user (IDU) and then transferring a portion of the given solution into a second syringe (that belongs to another IDU) by removing the plunger of the second syringe and squirting some of the solution from the first syringe into the second syringe. So-called 'Frontloading' involves removing the needle (rather than the plunger) from the second syringe and then drawing back the plunger in order to allow the first person to squirt the solution in. In some cases, the second person will be allowed to help themselves to as much of the drug as they want, they can do this by drawing back the plunger of their needle until they have the amount of solution that they want. It's a way of sharing drugs between users and the the terms 'backloading' and 'frontloading' are a description of how the drugs are loaded into the syringe(s) prior to being used. HTH.