In my racing years, I won many a sprint....assuming you are sprinting for a city limits sign on the flat, you would shift up to a bigger gear....jump out of the saddle to accelerate the big gear up to about 100 or 110 rpm and then sit down and spin up the rpms. If it is a sprint from a shorter distance out you might stay out of the saddle the whole time. When you first get out of the saddle you are pulling with your arms, and the bike is going side to side....watch some youtube videos of TDF sprints and you see how much the back end of the bike is flexing and moving....lots of torque from the legs and the arms