As I understand it, when you add an adjustment layer but make no changes, then do a blend mode on it, it has the same effect as duplicating the original pixel layer and doing the same blend mode. Hence this provides a handy way of doing a duplicate blend without having to double the pixel count.
When you duplicate the background image and do a linear burn on the duplicate, you get a general image darkening, as I'd expect. But if you add an adjustment (eg. Curves or HSL) and do a linear burn on this (without making any adjustments) you get a very different effect - it looks half like a hard mix.
Bug?