After shooting with the safe shutter technique, the images will usually be underexposed (but sharp!). You can then brighten and tweak the picture using photo editing software.
This single command will put the punch and detail back in to your (on purpose) underexposed photograph. Load the images into your favourite photo editing application, and use the Adjust Levels command to expand the detail that is compressed into the shadow areas.
Expand shadow detail - drag highlight bar to the left
The histogram display shows a graph of image detail. Most likely, you wll see most of the action towards the shadow (left) end of the scale.
To push the detail back up, you can drag the highlight slider towards the left. Preview your changes and keep going until the brightness is more to your liking.
Experiment with moving the midtone and shadow sliders, since you have your originals backed up you can always start again.
Noise and Contrast
You will notice increased contrast and noise in your digital-pushed images. This is the eternal trade-off between film sensitivity and quality:
The lower the light level,
the more noise you will get when pushing.
Remember thre are many other image processnig techniques that can help smooth and refine the image. Instead of fighting the noise and contrast, use it for effect!