I do know at least in my sons case that he loses about 3-4 mph when he slide steps versus a full leg lift. When he slide steps it is more lineear to home plate and I have noticed the arm does not get tweaked back as far due to the hips being more open at initial leg lift. I always dread when he gets runners on first because he gives up MPH. I always tell him to use more leg and get more rotation out of the hip which allows his upper body to get more initial windup. I have noticed that he throws the hardest not from the windup but from the stretch with a runner on second base. Fronm the video I watch it appears that when he gets a runner on second he gets a higher leg lift, more torso rotation and also more of a rotationl drive to home where the front leg comes down in more of an arc out around short and third base side then towards home at the last. He also seems to use this roatation as a leverage because right before he lands he brings it back and then straight towards home plate almost as if he starts to pull that side backwards against the direction of his rotating hips.