I was watching videos of F-35B taking off and landing vertically on ships when I realized that when transitioning from horizontal to vertical flight mode (I don't know the exact vocabulary to describe that), the horizontal elevator moves to "stick pushed full forward" position. The elevator is also put in that unusual position when transitioning from taxiing to a vertical takeoff. I know that the elevator position is not significant in vertical flight as attitude is controlled via non-aerodynamics means. Thus I wonder why this control surface is then put in that position for that flight phase instead of staying in its previous position or in a more neutral position.
Vertical flight needs the big panel behind the canopy open, to admit air to the LiftFan® that it otherwise covers. While that's open, in the transition to horizontal forward flight it would pitch the nose up. The deflected elevators might compensate for that, although the downward-vectored thrust at the rear of the airplane compensates more effectively during slow forward flight (as shown in the video in Anonymous Person's comment.)
The AV-8B lacks such a speed-brake-like protrusion, so it needs no elevator deflection to compensate.
The Yak-141 has what looks like a smaller panel. Photos of it hovering show no elevator deflection, but the far-aft flaps are strongly deflected, perhaps for the same purpose.