It requires a shirt, sweater, or jacket with long sleeves. Take your entire arm out of one of the sleeves and discretely place that arm's hand on your chest or shoulder by your other arm (under the shirt of course).

Now take your other arm and connect it to the empty sleeve by inserting just your hand in the wrist area of the empty sleeve (fiddle around so it comes out looking like you are holding hands by yourself).

Lastly, the movement: push your concealed elbow up so it protrudes from the chest area (while still under the shirt) and your other arm down. Then reverse the movement so your concealed arm goes down and your other goes up.