In Darktable, multiple instances of exposure are usēd to mimic the 'dodge and burn' effect brought to digital image processing from the film darkroom or the effect of an on camera graduated ND filter. The masking functions are very powerful, there are many options:
- drawn mask
- parametric mask
- drawn and parametric mask combined
Drawn masks can be circles, elipses, paths (similar to lasoo selections), brush or gradients and these have hardness and opacity properties.
Each of the mask types can be modified with blend modes such as multiply, divide, addition etc. and a mask refinement which can add feathering, bluring, contrast and reduce opacity. Masks can contain multiple shapes which can be combined in union, intersection, difference and exclusion modes in the mask manager.
A mask can be copied or shared between modules, for instance using a circle with exposure to darken the image and color balance rgb to reduce saturation for a vignette effect.
A simple example would be to darken the sky of a landscape photograph, using a second instance of exposure with a simple gradient drawn mask. A more refined method might use a drawn and parametric masking, a path to select the sky which is then refined with a feathering radius of about 30 pixels and then further restricted by setting parametric masking with the colour picker.
In the case of the portrait, use the path tool in drawn mask and select the area just inside the boundary of the person, finishing with a right click. Hover over the mask to make sure opacity is high and feather size of the shape itself is low. Toggle the shape off (the arrow and dotted line icon next to the mask drawing tools) and display the mask (the square with a dark circle icon next to mask refinement) the image goes black and white and the mask is shown in yellow. Move the feathering radius slider till the mask fills out to the boundary of your subject. This method of feathering is superior because it is guided to fit the image I think, but can introduce artifacts. Revert to the normal view and move the exposure slider to increase the exposure of the masked area. If the transition from masked to unmasked areas is too harsh or artifacts appear, play around with the refinements.
As regards help and tutorials, darktable's website has a resources page with links to the very good manual in several languages and formats as well as links to some video tutorials. Personally I like Bruce Williams' stuff as he comes at it from a non computer geek / photographer / user point of view and freely admits when something new in the programme stumps him.