Well, swarf builds up quickly and makes a black/grey visual indicator on that stone. Looks like he's also using too much pressure. If I remember right, the only time you should use any pressure on a straight is during bevel set on the 1k. Your final strokes on a 1k should be just the weight of the blade, and 0 pressure on the 4k, 8k, and finisher should you opt to use one. But hey, if it works it works. Just 'cause he does it different than me doesn't mean he's 'wrong.'