run fast, gallop, as one pleases
騁 centers on moving at full speed without restraint: galloping like a horse, running fast, or acting freely as one pleases.
騁 combines 馬 (horse) with 甹, which likely contributes the sound and may relate to the idea of swiftness or freedom. The exact historical development of 甹 is uncertain.
A horse 馬 gallops freely, and the right side 甹 suggests unrestrained movement. Imagine a wild horse running as fast as it pleases across an open field.
For テイ, picture a horse galloping so fast it leaves a trail of tea leaves behind: tea -> テイ, and the horse runs freely.