Get into a Plank position, turn your arms in and push your elbows out to get on your forearms, which should be placed right in front of each other.
Switch the forearms (the one in front goes at the back and vice versa).
Push yourself up to the starting position and repeat.
Risks
The shoulder internal rotation (with the elbows pointing out) puts the shoulder in a position that can lead to impingement, so it is best to limit or avoid this exercise if you have limited experience.