Simple looking, yet very effective core exercise when performed correctly. I have learned to coach and cue this exercise by having the individual fully exhale 1 or 2 times while laying on there back to bring there ribs down so there back is close to the floor and not arching, before they start to lower there arms and extend there legs. Engage the core and perform a full exhale when arm and leg is extended which will make it very challenging. You can progress this exercise by adding small weight plates in each hand or using a band, as well as regress the exercise by just extending the legs alternating one at a time and then adding the arms in later when you are more competent.
Overall a great exercise that I like to use a lot myself as well as with clients.