When shift-clicking the output slot with more than an item in a stack in the crafting grid, it 'removes' one item from the grid, but then produces as many items as were in the original stack.

So a stack of, say, diamonds, would remove one diamond from the grid, but produce an additional 64.

This will not work if you only have one of the item, so items that cannot stack aren't affected by this as far as I know.

Happens in mod version 0.8.10 on Minecraft 1.10

okay, let's say I have a stack of Quartz Enriched Iron in the Refined Storage Crafting Grid.
http://puu.sh/q7r81/93b6cffb75.jpg
It has no output. If I shift-click the output slot...
http://puu.sh/q7rdc/ef2e514cc9.jpg
The number of enriched iron in the grid decreases by one, but a stack of it spontaneously appears in my inventory.

Does this also happen for items that HAVE an output? E.g. does it dupe when shift clicking to make a workbench for example?

It does not appear to do so with recipes that have an output.