Reverse Grisaille post is getting a fair amount of attention so I decided to revisit the subject.The above is an example of what it does, albeit an exaggerated one. In practice I probably would tone the 'after' down somewhat, at least in selected areas.
To recap the process covered in the previous post; make a copy of your image that you then convert to B&W using the conversion process to obtain the best tonal values without regard to hue (best being your intent and definition). Then drag that B&W "grisaille" image onto the original while holding down the shift key so that they auto-align and change the blending mode to "Luminosity". At that point it will almost always be too harsh and you will want to reduce the opacity to the B&W layer. The "after" in the example above is set at 80% opacity. While the grisaille layer heightens local contrast it also has the effect of slightly dulling the color so you may need to add a hue/saturation adjustment layer around +10 to 15% saturation to compensate.
So why do it? Can't you get the same increase in local contrast using other Photoshop tools? Yes, but... I find it easier to judge tone if hue is not present. Aside from that this technique gives you a lot of control to play with. For example if you save the layered file you can easily increase or reduce the effect by playing with the opacity slider on the B&W layer. Also you can alter the effect locally by adding a layer mask on which you can selectively paint out the effect wholly or partially by varying the opacity of your brush.
An interesting thing I have noted about this technique is that the grisaille both darkens the dark areas and lightens the light areas. Note the changes in local contrast in the sky and white water where it tumbles down the falls. Also the increased detail in the fallen tree and dark rock at the lower left. It acts like a form of HDR in that sense and seems to do so without creating noise. Another possibility is to use the "Lighten" or "Darken" modes on the grisaille layer for different effects. Those effect only the highlights (lighten) or shadows (darken) and subdue the color more but produce some potentially interesting effects.
If you try this and post results to the web, I'd like to see them. Leave a comment with the URL for your images.