In the photo above you can compare the two sears. The lower sear is unrelieved, the upper is relieved. The safety is on in both photos. You can see that, unrelieved, the safety bar is almost right against the sear. The receiver cannot be pulled back and the action cannot be hand cycled with the safety on.
The relieved sear is cut back (actually forward) sufficiently that the receiver can be pulled back far enough to cycle the action even with the safety bar up.