You know, I'm 52 years old and I don't think I've ever seen an up-close picture of yak that gave a good sense of scale. Those things are just cold-weather banthas.
I had a similar realization about bison recently too, those things are goddamned huge, bigger even than that mid-sized SUV. Apparently they can get up to 3000lbs in weight.
The nose ring is pretty much necessary. Bulls (in general and I assume for yaks its the same thing) lose their respect for humans the older they get. It can then happen, that they try to attack a human to demonstrate, that they are stronger (which they are). In this case this nose ring is the only way you can save yourself. Grab the ring, turn it and suddenly a raging bull turns into a quite submissive animal.