Easy way to remember is that all loose ball plays have to occur behind the LOS. Fumbles beyond the LOS are treated as a running play ,with the fumble marking the end-of-run. Same is true with a backward or illegal pass beyond the LOS. Consider a runner off for a 50 yard gallop and ,as he is about to be tackled, he pitches the ball forward to a teammate. The surprised teammate drops the IFP, killing the play. The basic spot would be the end-of-run (spot of IFP), we wouldn't go back 50 yards to the previous LOS.
Hope this helps.