A rider’s stubborn insistence on moving forward creates tension in the chain that pulls the gear on the back wheel. The twisting (torque) force goes from hub to rim to ground and away he goes.
Since the wheel is such a sparse structure, it must manage this torque efficiently without flex, fatigue, or excess weight. Through the cleverness of tangential spoking, a rear wheel can transmit torque with ease.
Wire spoke wheels were in use for decades before tangential spoking was devised. James Starley is given credit for the first tangential spoke product, the Ariel bicycle of his Coventry Sewing Machine Company.
The Ariel front wheel has two (and the rear, one) radial levers among the spokes. From the end of each is a wire connection to the rim. Tightened, and the semi-loose spokes of each wheel become snug as they all acquire a slightly tangential angle from hub to rim. This angle makes torque transfer more efficient.