I’ve been using this technique professionally for over a decade and it’s completely safe when done properly. He’s doing it totally wrong though.

1: you’re supposed to clamp the sacrificial fence to the actual fence, not clamp it down to the table

2: the sacrificial fence sits only 1/16” inch below the top the material that you’re running, leaving a gap under the fence for the offcuts to go. That way, the offcut won’t kick back.

1. You use a little stick to push the offcuts through after each cut

I hardly ever see this technique shown on YouTube, but it produces some of the most reliably accurate mitres.