Techniques that utilize deep-tissue/deep-muscle massage are administered to affect the sub-layer of musculature and fascia.  These techniques require advanced training and a thorough understanding of anatomy and physiology.  The muscles must be relaxed in order to effectively perform deep-tissue massage.  Otherwise tight surface muscles prevent the practitioner from reaching deeper musculature.  It helps with chronic muscular pain and injury rehabilitation and reduces inflammation-related pain caused by arthritis and tendinitis.  It is generally integrated with other massage techniques.

(Source:  www.massagetherapy.com/glossary/index.php)