Using Virtual Medicine, Stephania Bell explains the details of Patrick Mahomes' injury. (3:30) KANSAS CITY, Mo. -- Patrick Mahomes, the Chiefs' star quarterback, posted a short video clip Wednesday of himself in a training session where he dropped back and threw the ball, the first public footage of him doing so since he had surgery in December to repair the torn ACL and LCL in his left knee. Mahomes posted the four-second clip on his Instagram account. His movement, doing the dropback at a moderate tempo, demonstrated the progress that he has made during his rehab. Wearing a compression sleeve on his left knee, Mahomes was able to plant his left foot -- allowing him to put some of his weight on his left knee -- and throw the ball, which appeared to be an intermediate pass inside a training gym, while executing the proper follow-through. "Day by Day!" Mahomes wrote his Instagram story.…