Move the trail hand like skipping a rock
The speaker describes the trail hand as easier and non-rotational, moving like skipping a rock. From the proper position of the body, he tilts the body, stabilizes into the leg, and rotates with a bent arm and a hinged wrist, skipping and extending without rolling anything.
Channel: Todd Graves
Watch official source at 8:31Why this context matters
The 'skipping the rock' analogy describes the trail-arm motion through the swing.