i think you see how 8 is placed. after that, 2 must be in the middle row of bottom left square. so 2 can be only in the bottom row of bottom rght square
i didnt watch remaining of video, he maybe explains but 1 in the middle bottom square and 1 in the upper right square makes 1's place just the near of that 2.
so only place left for 2 is there.
i hate english ^^