This is the shortcut I'm thinking about, 4B. I think the one WB described is 4A:
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4A (300) (Ascending or descending) The “Peach Mango” shortcut leaves the Main Trail at Mirror Lake, just after the Mirror Lake sign, and ascends sharply up on nice slabs, to rejoin the Main Trail before it heads up and west over the long bench toward Trailside Meadow. There is a big bottle of orange drink under a tree about a third of the way up. Here and there you can see the Main Trail a couple of hundred feet to your right, switching gently back and forth while you continue your upward progression.
4B (400’) (Descending) The “Tin Pie Plate” shortcut saves only a few minutes if that, but it is a refreshing change from the monotonous Main Trail, and there is a nice class 3 section near the bottom. If you are in a hurry, take 4A down instead.
On the bench overlooking Mirror Lake, the Main Trail gains 5’ in elevation, loses it, then gains 5’ again. 100’ past the top of this second rise, slant sharply left for 50’ and drop to a sandy ledge containing several small pine trees. Turn hard left (west) to enter a now obvious couloir, and descend it for ~300’. Upon reaching a ledge from which further progress seems unlikely, turn right. Passing a duck with a tin pie plate is the key. Descend class 3 the rest of the way, ending about 100’ above Mirror Lake. Now traverse to the Main Trail.
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Description of "sandy ledge containing several small pine trees" makes me think of where the SAR Rap picture was taken.
