This ^^^ is good internet advice. My thought is wrist is relaxed and passive and the site above seems to show that - at least that is what I see. And, yes there is extension up and thru the impact zone.
But, I do disagree with the site that it is straight up the back off the ball. You are rotating your arm into contact with your core rotation hips and shoulders. The pattern of you hand and racket head is an eclipse if viewed from above. There is an element of across in the swing as the eclipse will begin to pull to the L for a right handed player around the impact zone. The has been discussed, debated, and argued too often here, but I think the correct thought is the racket head moves up, thru, and across with a passive wrist - no fancy brushing, no yanking back with hand, just a smooth stroke.