a.) Look at the head position and the breathing. Not much movement there. Head is neutral and you can barely see his goggles coming out of the water. Basically, it is almost like he is sneaking a peak in there. :)
b.) Watch how smooth and relaxed he looks. This is the key to fast and efficient swimming. Looking at the underwater video, I'd guess this is from his 1500m swim (I guess you can see that on his time :)).
c.) When underwater, check out how he is grabbing so much water with his forearms up front and then pushing it behind him. He is also stretching out of his shoulders forward and finishing his stroke all the way below his butt.
d.) There are more key points that you can get out of the video, but these are the main ones. Feel free to comment on this and give us your opinion.
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