Upper control arm is the V part of a triangle.
The ball joint/spindle is at the intersection point.
If you shorten the rear leg, or lengthen the front leg, the point where they meet moves, in this case back.
That provides more caster.

Some of the old SS cars, like mine, had theirs done by Jack Arnew - his deal was to widen (lengthen) the front leg.
Side effect was the front wheels had consistent camber, whether on the ground or in a wheelstand.
In my car's case, we could get over 7 degrees of pos caster.