Not really a swim drill, but more of a focus on proper catch and pull technique to engage the lats and not the shoulders.
'Pull' Drill - Focus will be on learning to 'catch' the water with your forearm and pull yourself through the water. If you feel your lats (the muscles under your arm pits that go down the side of your back) engage, then you are starting to get it. If your shoulders are tired, then you are still using too much shoulder and not enough lats.
VIDEO: The 'Pull' Drill
Not really a swim drill, but more of a focus on proper catch and pull technique to engage the lats and not the shoulders.
Video - Pull Swim Drill (727 downloads)
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