I've found this expression on the Natural Selection 2 fanpage: https://www.facebook.com/NaturalSelection2/posts/10152301360338105
He's just mentioning to notice the soldier falling at 00:14 in the video.
"Dat" should means "that", I've only found this word in colloquial Internet language (such as in "dat ass").
jinawee is almost correct.
Rather than referring to a soldier falling down, this is referring a bass drop at 00:14 in the video, where the background music went "wub-wub-wub" for a moment.
To add a bit more detail: Each time I've seen the colloquial Internet usage of "dat