Stirring the curds-whey mixture facilitates heat transfer during cooking, prevents the curd pieces from fusing and promotes syneresis by encouraging collisions with other curd pieces and the vat wall. It is important to stir gently after cutting to avoid curd shattering (and thus yield losses); indeed, some cheesemakers leave a 5-10 min "healing time" after cutting before starting to stir.
In Cheddar-type cheeses, the curds-whey mixture is stirred and cooked to a desired pH (e.g., 6.1-6.2) but in many varieties (e.g., Emmental), the whey is drained at a target temperature. In a few varieties (e.g., stirred-curd Cheddar or Colby cheese), it is normal to stir the curd pieces after drainage ("dry stirring") which also promotes syneresis.
Note: Figure shows the effect of stirring (solid curves) and no stirring (lower broken line) on percentage syneresis as a function of time afer cutting.