Deglazing is adding liquid to a hot pan to dissolve the fond — the concentrated layer of browned proteins, caramelised sugars, and rendered fats stuck to the cooking surface.
Three forces work together to release fond: thermal shock, solvent action from acidic liquids like wine, and mechanical scraping. Once in solution, reduce the liquid by simmering. Finishing with cold butter whisked in off heat creates a glossy, emulsified texture.
The fond tells you about your sear. Pale gold fond means light browning. Deep mahogany means intense Maillard development. Black fond means something burned — do not deglaze it. The professional finish: reduce by half, then whisk in a tablespoon of cold butter off heat.
Deglazing a pan that is too hot. Using too much liquid which dilutes the fond. Not pouring off excess fat first. Adding dairy directly to a screaming hot pan — it will curdle.