The motion is a lot like strumming a chord

Notice the circular motion of your right hand.  This is what people always say is “like buttering toast.”  If you make it more minute, it’s going to be a bit more like turning a key in a door, or even more like turning the key in your cars ignition.  Even more minute and it’s like jiggling a key in a finnicky lock.

I think the best advice that I’ve heard out there is that the motion of alternate picking should feel a lot like the motion of playing rhythm or strumming chords really fast.  This is the most intuitive as most likely you’ll want to switch from a wider stroke to a more refined one and you don’t want to worry about changing the way you rotate, angle, twist, or hold the pick.  The other reason I like this is that if the stroke feels akin to strumming, then you’re more likely to feel rhythm and groove while playing.  If you watch Paul Gilbert or Guthrie Govan they sort of have this groove about them when they shred, bobbing their head and even beginning their approach toward the strings as though they were going to churn out a funk rhythm.