What I like to tell my clients the only pain I want them to feel is the "It feels so good, it hurts." The best way to describe this type of pain is the pressure that the massage therapist is giving is the correct amount releasing the tissue fiber from it's tightness. You as a client can literally feel the muscle, tissue, tendons, or ligaments being released. What I think is that the pain receptors are firing back at the brain signalling it is being engaged but not being damaged which can send a confusing response. If the pain threshold goes past this feeling I recommend my clients to speak up so I can back off on the pressure so their muscles don't start fighting against me.