I have sparred several TKD folks that did that. I actually just thought it was poor form, LOL. I was shocked to find out that they had been taught to do that. Sine wave is very easy to time, IMHO - at least in sparring.
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"In case you ever wondered what it's like to be knocked out, it's like waking up from a nightmare only to discover it wasn't a dream." -Forrest Griffin