Usually just called "poaching", ski poaching refers to skiing or riding in closed areas.

Although closed areas are usually closed for good reason (ie: avalanche risk, etc), they are also often closed because they are deemed too dangerous for tourists (read: lawsuit). However, locals often enjoy and know how to ride these areas "safely" and will do so regardless... hence poaching

When you eat dirt for the first time and leave your family to build a shack in the woods