Don’t you need a flat rock under the bottom of the brace? If you nail a deadman to the bottom outside of the post and another about half way up on the strainer side of the post, they will help hold the post in the ground. You can also drill a hole in the bottom of the brace and drive through the post into the ground to keep the stock from working the bottom of the post out of line with the direction of the tension on the fence.
I like to leave the brace post square and cut a recess into the fence post. You just need a nail to hold it in place while you are completing it. After tightening the wire it will wedge itself in tight and never split from the big nail holding all the strain.
Yeah, he made several basic mistakes. Not using a foot on the main post (he mentions it but doesn't do it), not using a pad or footing for the brace post (to keep the brace from either sliding down the fence line or going into the ground) , using a nail to secure the post to the brace (it will split the brace post over time, it would have been better to either use a mortise and tenon joint or at lease cut a notch in the main post and left the end of the brace post square) . Less detrimental but also not optimal is only using one brace wire (should double loop or use cable or heavier wire). Relying on a staple to attach the wire to the bottom of the brace post. That is where he could have used the nail or at least a slot in the wood. I think the guy had the basic idea right (using high tensile plain wire and dead man brace) but just didn't have the real world learnings of fencers who have built thousands of miles of fences over the last half century.
The wire w/strainer on the bottom is pulling the brace pole down to the ground and at the same time it is pulling it back towards the bottom of the post, this is to stop any movement of the fencing from pulling the post down the fence line.
