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
"I got caught poaching by patrol"
- meaning "I was caught snowboard/ski poaching by the ski patrol"