Nitrite Pickling Salt is ordinary salt with .4% of Sodium Nitrite (NaNO²) added. Generally, the salt used is non-iodized, but some brands might have iodine in them.
Nitrite Pickling Salt helps to fix colour, and preserve food. It also helps protect against botulism.
When used, the nitrite forms into nitric oxide, which is the colour fixative.
History Notes
In 1899, it was discovered that it was nitrite rather than nitrate which actually contributed to preserving.