It seems you still can't wrap your head around the triangular shape. We assume from the beginning that in the reference frame of the light-clock, the light pulse travels up and down, hitting the mirrors over and over, like this || for lack of better graphics. Now, considering a different reference frame that is moving horizontally with respect to that very same light clock, (like you are driving past it at constant speed in your car). If you naively think the light still travels a vertical path like this || then the light pulse will miss the mirror, because in your car reference frame the mirror moves to the right by the time the light arrives there. So, as he indicates in the video, we know that vertical path || cannot be correct, because then you have created a contradiction. Think about it, if a light clock is working perfectly in its own reference frame, and you drive past it in your car, it must continue function properly. So we know the light pulse cannot miss the mirror, and the path must be something like this /\/\/\/\ obviously. This should be as easy to understand as if you dropped a ball on a uniformly moving train. You would say the ball drops straight down, and bounces back up, like this ||. But someone watching from the train station would say that the ball also has motion in the direction the train is moving, so the path is more like this \/\/\/\/ (although there would actually be a parabolic shape in that case, due to gravity causing the ball to accelerate, rather than move at a constant speed as the light pulse does. I hope this helps, even if I am not on your list of approved posters.