Use it when things start hurting too much. Maybe it's going too fast? Maybe the method goes against what you usually do?
Workshops use tools and methods that are different from what people are used too. It's normal that your workshop participants feel at times a bit uncomfortable. But when it starts hurting they should express it in order to avoid to become ticking bombs.
This card can be used by workshop participants when they start too feel just a slightly too much uncomfortable.