This variation on the missionary position illustrates how you can keep your knees on the bed as additional support, similar to the way that knees can be used in a push up to take some of the weight off your upper arms. While our wooden models can't quite do it, there are lots of options of how you arrange your knees (close together, farther apart) and these can change the feeling and movement in the hips of the person on top. The partner on bottom can also adjust their legs depending on where your knees are.
This images also highlights one of the benefits of the missionary sex position, which is the amount of physical face-to-face contact it provides.
