Keep the trail foot on the ground to stabilize
The speaker keeps the trail foot on the ground as a stabilizer that limits motion. It does not stop the hips but only allows them to go so far, which keeps the spine back and maintains posture. He notes that if the foot comes up, he gets extension.
Channel: Todd Graves
Watch official source at 12:07Why this context matters
Addresses a recurring debate on his channel about whether the trail foot should stay down.