The most physically demanding stroke, the butterfly features the simultaneous overhead stroke of the arms combined with the dolphin kick.

对身体条件最苛求的泳姿，蝶泳的特色是两臂同时越过头顶结合海豚式打腿。

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It often goes out if I use the backhand-overhead-stroke.

如果我反手肩上击球，球常常出界。

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It always goes out if I use the backhand-overhead-stroke.

如果我用反手肩上击球，球老是出界。

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To check if it actually works as advertised, we set up a a video camera to film the stroke from overhead, then compared the plotted result with the stroke path displayed on a smartphone, as shown in the video below.

ENGADGET: 3BaysGSA Putt: a Bluetooth golf gadget that puts eyes in your putter (hands on)