Straining vs Relaxed

Well, I suppose I’m just lost as to what to do here. To keep it simple, I run much faster when I overexert myself and slightly strain to get the most out of my movement (.2 to 30m, .5+ to 60m). So when I do max effort speed work, should I go hard and strain a little bit, running at max speed…or should I practice relaxing through the movement, training (atleast temporarily) at submaximal speeds?

And if its important, I am pretty fast [today I ran 4.15 30m, PB is 4.13 ET from first motion]