The end area method uses the wedge concept, distributing the volume longitudinally along the cross section, tapering upstream and downstream to zero Δz (elevation change) points at the bounding cross sections. But unlike Simpson's rule, the end-area method just assumes linear transitions. So:
The model converts mass to volume and spreads the volume change upstream and downstream from the cross section in a "wedge", to determine the area change at the cross section. An exaggerated bed change is shown at river station 2, in the figure in Volume Change and Area Change Conversions. HEC-RAS then converts the area change into cross section change.
Once HEC-RAS computes sediment surplus or deficit and applies the limiters, it computes a final mass to erode or deposit from the control volume. HEC-RAS adds or removes mass by adjusting the cross-section station/elevation points.