With certain underexposed photos some of the detail hiding in the darker areas of the photo can be recovered. Avec Photo can check your image to evaluate if it is possible to correct this problem. Like overexposure, some areas of the photo may be beyond recovery and will remain black. This can look strange if the rest of the photo which has detail has been brought up to the correct exposure.
The photo below has been underexposed
In the digital darkroom Avec Photo can (depending on the photo) correct the underexposed photo.