Relaxing or trying to increase the effort?

Towards the end of a 400m/800m races should you try and increase your effort to maintain speed or should you just relax and focus on maintaining good technique.
I’m wondering because towards the end of a race I feel as though I can increase my effort but it just seems that soon after I begin to tighten up, lose form and slow down. However, when you watch old videos of Seb Coe for example you can see a pronounced increase in his arm drive as he enters the final straight allowing him to maintain momentum. Wilson Kipketer also has a dramatic change in his arm action entering the final straight. Someone like KD Robinson just appears to relax though as if he knows that trying to increase the effort will be to no avial.
Any thoughts?

To go fast you must stay relaxed. Especially in the neck and shoulders. Your shoulders should be down and back. Your jaw should be slightly bouncy and your technique should be perfect. You cant do this while you fight your self.