A pin wash is a wash applied with a fine brush point directly into recesses — panel lines, bolt heads, vents — rather than slopped over the whole surface. The name comes from placing paint with a pinpoint.
It is the controlled sibling of the all-over wash: same shading effect in the details, far less cleanup, and the flat surfaces keep their clean base color. Gloss-coat first so the wash flows and wipes cleanly.
See it in context in our weathering & effects guides, or explore the tools hands-on in The Bench.