Time Trial / Time Trialling / TT

if your riding a course with a head wind and a tail wind section which is totally flat is there any reason why you should vary your effort at all? You want to minimise your total effort, the only serious resistance to your movement is your speed relative to the air so surely you would aim to ride steady throughout so you were slower into the headwind and faster with the tail wind. Is there any physical advantage to riding harder into the head wind to avoid going slowly at that point?

Yes, because the headwind section is harder and will take you longer to complete. So, says it's a 10mi, then you spend 60% of your time riding into a headwind and 40% of your time flying back with a tailwind. So, it's better to ride a bit harder into the headwind to get it done quicker, even if you ride the easier, tailwind section slower.