About 25-27 seconds in you see the dreaded 'spiking the elbow into the spine' See for yourself how effective it is. Also, at around 1:44 Royce also tries the 'dim mak' of elbows to the spine.

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This shows exactly why this won't work. By the time it was thought to try it, the guy in yellow was already too far off balance, and being driven back to hard to make an effective strike with his elbow.

NOW, if the BJJ guy stepped in and stopped just short of taking the other guys center, and waited, then an elbow shot would work because you would be free to drop as many as you want from a solid position that allows proper power generation....of course, I don't know a BJJ guy on the planet would just stop and let you do that to him.....but if he did, then it would work.