While a rig crew was nippling down cementing head following a casing job, an employee was suspended from a man-rider hoist. When the cementing head was loosened, it twisted around and the valve struck the suspended employee on the knee, causing a contusion. The employee was examined and returned to work. A second employee on another project was inspecting the top drive while suspended from the hoist. He was holding himself in a position against the pull of gravity and his foot slipped. He swung into the brace that was supporting his weight and struck his inner thigh, causing a bruise to the ball of his hipbone. The injury resulted in a lost time incident.