Also known as deep soft tissue massage (DSTM), this simply means applying massage or acupressure techniques with sufficient pressure to access muscles and other tissue that lie underneath the surface tissues. This is not a modality, as there is no specific set of skills or a common approach to DSTM - it simply implies that the massage is directed with more of a remedial intent rather than simply to relax the client.
Most people ask for a deep tissue massage to ensure that the therapist is aware that they like to feel a considerable amount of pressure during the massage. From a western massage perspective, DSTM usually involves trigger point therapy - whereby what the client often feels as 'knots' in the muscle are directly manipulated by the therapist.
Whilst it is not a given that a deep tissue session will be painful, it is often the case. Many clients describe this as a good pain, others just see it as a necessary downside to having their particular problems treated. It is important to be aware that DSTM does not imply that the entire session will be painful or even deep. The DSTM techniques may be mixed with relaxation techniques, stretching, or any other set of skills possessed by the therapist.
As DSTM is not a modality in itself, these are as for a regular massage therapist. Having said that, it takes a considerable amount of expertise to apply DSTM effectively and safely. It is best to seek a deep tissue massage from a practitioner such as a Remedial Therapist, Myotherapist, Rolfer, Shiatsu, Bowen, Osteopath or other highly skilled massage practitioners.