Depth is how far into the layers a stroke reaches, increased only after the surface has warmed.
In 📝Deep Tissue Massage depth is earned. A first pass stays superficial; later passes sink as the tissue lets them. Adding depth by adding speed or by ignoring a brace is how the work becomes painful without getting deeper. Depth is a dose variable, not a diagnosis of how "tight" someone is, and not a score of the practitioner's strength.
Clients feel the difference as a stroke that arrives rather than a stroke that crashes. If the muscle guards, depth is dropped and the surface is warmed again. Sharp, numb, or panicky sensation is a stop.