run, race
騳 centers on rapid forward motion, specifically running or racing, as suggested by the doubling of the horse component.
騳 is formed by doubling the horse component 馬, likely to emphasize the idea of horses running or racing together.
Two horses side by side: when you see two 馬 together, they are racing or running at full speed.
For トク, imagine two horses racing to the finish line, and the sound of their hooves is a rhythmic 'toku-toku': toku -> トク.