The Men's coat

Hi there – naive question but I don't think I understand what opposite bias means, or what the issue is with it.

Is it to do with how the two pieces of the collar canvas are sewn together? In which case I need to be wary of exactly how I cut out the pieces from the canvas?

I'm also a little confused as in the video you talk about stretching opposite corners of the canvas but it looks more like one whole side is stretched and the other shrunk.

Up till now I've naively copied what's happening in the video but I'm not sure if I need to make sure it's the right way round, or perhaps not even do it at all. Is it an option to cut the undercollar from a single piece of canvas, if we have a piece big enough?