To maximize your speed, you want the maximum amount of power in your sails, at the same time as keeping the boat as flat as possible at all times. The hiking strap, or toe strap, in the center of the cockpit is there so you can hike or lean out of the boat as far as possible, to counteract the force of the wind in the sails, to keep the boat level.
To increase your ability to keep the boat flat, put your feet under the footstrap, lean back as far as you can, and straighten your legs. This is where your fitness (strength and endurance, mainly in your thighs and stomach) come in. The longer you can stay out in the fully hiked position, the faster you will be.
Check out the great video below that goes into detail on how to adjust the toestrap for different angles of sail.