Hopefully this picture of what you want to look like off the line comes through
Start is everything damn near... the other stuff you NEED but if you don't have a good start you can look slower than you really are. End up with a 4.8 instead of a 4.5, etc.
This is how I believe it works... I could be wrong...
if you're right handed (stronger side of body):
right toe directly behind the line.
left toe staggered behind your right heel...
kneel down and put your left hand parallel to your right foot
Most of your weight should be on your hand and the right ball of your foot.
Right hand, stretched waaaay back.
RIP that right hand forward, explode off the right ball of your foot and jerk that left arm backwards, while bringing that left knee damn near to your chest.
It's only forty yards so there's no reason to stand up or hold stride or any of that other shit... just get to top speed as fast as you can and try to push past even that.
ARM SWING is vital. Keep your shoulders square and pull your elbows BACK don't let your elbows come out too much past your body.
---for every action there is an equal and opposite reaction. Try this, Jerk your arm (in a running motion) forward as fast as possible...it will make you lean backwards and that is what you do NOT want to do.
Jerking your arm (think elbows really) backwards puts you at a proper forward angle.
Allow your stride to open up naturally that will only come with practice.
Because its a shorter distance your strength is the overcoming factor in the speed that you can and will attain.
How you train these I will leave up to you but I particularly like training with a conjugate mindset. Mind you EVERYTHING needs to be strong, but these tend to have the most importance.
Posterior Chain- This will determine your top speed. (Snatch grip deadlifts on box, rack pulls with toe on weighted plates, cleans, box squats, jerks)
Quadriceps (most notably vmos)- will determine how quick you are off the line (Bulgarian squat, deadstop front squat, lunges, stepups, jerks)
Abdomen- Will keep your spine neutral and from looking like a flopping fish.
(Dragon flags, hanging pikes, weighted situps with weight behind your head, rotational stuff)
Traps/Rear delts- Arm swing baby!
(rack pulls, power cleans, rear delt flyes, dumbbell shrugs, chest supported rows, jerks)
Hope I helped.