I suspect the coffin curve relates to power/drag. At normal cruising speeds the drag and the power are in equilibrium and constant speed is achieved. If you reduce power, the drag slows the aircraft, but a slower aircraft has less drag and you achieve a slower cruise (and a slightly increased angle of attack). Eventually, as you reduce power and you try to keep the aircraft flying, the drag actually starts to increase as the airspeed decreases. At this point you've got a high Angle of Attack (and hence are about to stall) and you're in trouble unless you're a few inches above a runway or a few hundred feet in the air - and hence the coffin curve. (There are other threads describing how to land, and the concept of stalling the aircraft onto the ground with which you may or may not concur.) At "normal" flight speeds the stall occurs at a fixed AoA irrespective of speed.