I think around 1:30 he demonstrates external rotation.
Basically, rotating/pulling the forearm away from the body, whereas internal rotation has you rotating your forearm towards and into the body.
If you look at a throw, in the windup you'll have some external rotation and in follow through you'll have some mild internal rotation.
Putting your upper arm at the side of your torso, then bend your forearm to 90 degrees. Keeping your upper arm at your side and just roate, if you move your forearm towards your body/tummy/front it is internal rotation. If you move your forearm away from your body it is external rotation.