As airflow is brought to the desired velocity, sensors measure forces, such as lift and drag, on the test article. Lift is the force on the wing opposite the force of gravity.
Lift holds a plane in the air.
Drag is the force on the wing in the direction of the airflow. Drag is what an engine must overcome to move a plane through the air. Based on the measurements of these forces and the known relationships between the test environment and actual flying conditions, accurate predictions of real-world performance can be made.