A session of trigger point therapy usually involves continual communication between the therapist and the client to ensure there the client doesn’t experience too much pain when the therapist finds and presses on a trigger point. Typically, a pain scale of 1 through 10 is used to help the client communicate the level of sensation that they feel when the therapist finds a trigger point.
You can find more detailed information on trigger points on one of my favorite resource sites at www.painscience.com. There’s an ebook available for purchase called, “The Complete Guide to Trigger Points & Myofascial Pain“. I recommend you purchase the book to help you understand more about this subject.