Correcting 40-Yard Dash technique can be challenging because of the high speeds. You might catch some minor flaws, but you have to see yourself (or your athlete) actually sprinting in order to break down and address specific issues. (Check out this full 40-Yard Dash training program.)
Filming the 40 from the front and side can provide a great deal of insight into technique issues that may exist in each phase of the sprint. You can then attempt to correct flaws and watch subsequent sprints to assess progress. As you refine your technique, you will increase your speed by eliminating unnecessary energy waste, so your stride efficiency will propel you forward.
Create a checklist to assess each technique issue and note how to correct it. Record your notes after each sprint to see what still needs to be corrected.
Below is a checklist of many common technique issues and corrections. Use it as a guide for your personal 40-Yard Dash training.