The finishing dry-and-heat. It drives off the last moisture so the tea keeps, and it shapes the aroma. A heavy firing gives a toasty character, a light one keeps the fresh, green notes. Much of why the same leaf tastes different from shop to shop comes down to this step.
In more detail
Crude tea has already been dried, but still holds around five to seven percent moisture and its aroma has not settled. The final firing brings it down to roughly three to five percent and builds the aroma that heat creates. The same leaf from the same farm becomes a different product depending on how hard it is fired — in Sayama the heavy firing is itself the regional signature.