The meat consists mainly of three materials: water, proteins and fat
When a strong source of heat is applied to the surface of the meat, the protein coagulates and aggregates to form crust. This crust is the result of the Maillard reaction also responsible for crusted bread.
The presence of heat and water, will make the collagen (the proteins present in meat) degrades to form a gelatin. This the reason why the meat is removed and this brown crust appears at surface. But if the meat is warned to long, the evaporation of the water associated to the coagulation of the other proteins in the surface, will have for inverse effect to dry and harden the meat. It is necessary here all the art and the experience (experiment) of the cook to find the equivalence completed between these two opposite phenomena.