Most powerful leg is forward in the blocks.
Front block is just less than 2 of your foot lengths behind the starting line. (As you measure, your toes can be up on the block).
The rear block is just less than 3 of your foot lengths behind the starting line.
Note: If you're starting on a curve, put the blocks on the outside of your lane and angle them inward.
"On your marks"
Stand in front of the blocks and back into position.
The front knee is just behind the starting line.
Hands are shoulder width apart. Your fingers are just behind the line.
Shift your body weight forward. The goal is to have 70-80% of your weight on your hands.
In the "set" position, your hips are slightly higher than your shoulders.
Your front leg is at a 90-degree angle.
Keep your back flat.
Head down, but your eyes looking about 10 meters down the track.
Clear your mind.
If you think of the gun sound as "bang," start on the "B" of the "bang," not the "G."
Powerful arms. Powerful legs. Powerful knee drive.
Driving forward, you reach your full height at about the 4th or 5th step -- in the hurdles (about twice that for sprinters).