Don’t get to hung up on the curvature.
To try and figure this out imagine a symmetrical wing.
Symmetrical wing (flat board) at zero angle of attack creates zero lift.
Now angle the back end down, you’ve created camber aka curvature > “positive lift”
Now angle the back end up, you’ve created camber aka curvature but in the other direction > “negative” lift which is simply lift in the other direction.
Another way to see this is by looking at the rudder from the top.
Deflection left creates a lift vector that pushes the tail right.
Deflection right creates a lift vector that pushes the tail left.
Not a perfect explanation but it works.