In this variation of the missionary position notice the position of the legs. A, who is on top, isn't directly on top, but is actually over to the side of B a bit. Because this is a still photo what you can't see is that this off-center position allows B to raise their leg (or legs) and be penetrated from a slight angle.
This becomes similar to a scissors sex position where you and your partners legs are entwined but at right angles. If one or both of you has good movement in your legs this variation can provide an opportunity for creating movement during intercourse simply by swinging your legs up and down, or back and forth.
