It is obvious that on wide pieces, like the sides of a jewelry box, where the width exceeds the capabilities of the miter fence to hold it steady, and/or the piece is wider than the plane iron is tall, the Donkey Ear will come into play.
Yet, those two criteria are not the only times where I would choose to use it.
Take a piece 3/8” thick and 1 1/8” wide, saw a bevel on the end, and consider refining the “carcase miter” on the shooting board.
Also, consider the vantage point of what the plane iron sees. With the workpiece against the Miter Fence, it sees 1 1/8” of blade involved.
Take even a fine shaving in that situation and you will feel the iron wanting to pull the piece along with it, much more so than on a thicker piece which offers more resistance. Thinner pieces are more liable to deflect under the pressure of the cutting action.
Place the piece on the Donkey Ear and the iron sees (at 45 degrees) ½” of material to plane.
The width of 1 1/8” offers plenty of resistance and there is no tendency whatsoever to pull the piece along. It is pushing against it at ninety degrees. As long as you can back the fibers at the exit, you are good as gold. With molded pieces I sometimes use blue tape or a knife line to help support against blow out.
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