Static vs Dynamic
A 7-iron may have 34 degrees of static loft on the rack. At impact, a tour pro will lean the shaft forward, presenting roughly 24 degrees of dynamic loft to the ball.
Shaft Lean
Forward shaft lean reduces dynamic loft, which lowers launch angle and increases ball speed (better smash factor). Backward shaft lean (casting/flipping) increases dynamic loft, resulting in high, weak shots.