Darkroom is a powerful photo editor for iPhone, iPad, and more recently, Mac. When using its editing features, it has a couple great tricks for resetting individual edits and previewing how your photos look without them.
To reset an adjustment slider: Double tap the slider knob and it’ll snap back to zero
To see your photo without an adjustment: Say you’ve made a lot of adjustments and you’d like to see what each of them brings to the table. Tap and hold on the adjustment title (Contrast, Temperature, Highlights, etc.) to temporarily remove its effects from the photo.