I trained under a sifu who was a student of Chris Chan in San Francisco. Chris was a student of Yip Man.
Chris' approach is very heavy on power development. They train Sil Num Tao a lot, and do the first section slowly, with a lot of tension and force. The way my instructor would have us do it, we just breathed normally as we did this.
I started to time my breathing with the movements. As I would extend, I would breath out slowly, then exhale sharply at the wrist flick at the end of the movement. As the wrist grinds around, I would inhale and relax. Then, as the arm is withdrawn again, I would breath out slowly as the arm is pulled back in slowly, with tension and force.
In short, you exhale on the heavy-effort parts, and inhale at the relaxation points, regardless of which direction your arm is actually moving.
I talked with my sifu about it, and he believes it is a better way to develop the power, rather than just breathing normally. I think he teaches the other way because he teaches at a community college where he gets a whole new crop of newbies every semester, and it is too challenging for most of them to grasp.
There are potential problems with this method, however. If it is overdone, it can lead to hypertension and high blood pressure and heart disease later in life. There is a saying that the masters who train arts like Goju-ryu, Hung Gar, and Wing Chun, that use this kind of tension and breathing, often die at a fairly young age. Of course there are always exceptions: Hung Gar grandmaster Lam Jo lived well into his 90s (I think he passed away within the last few years).
I just try to be moderate in how much I do this, and I always finish my training with tai chi chuan, to dissipate any ill effects of the practice.
I know that other schools don't train it this way. This is, to my understanding, a result of Chris Chan's focus on power. I believe his brother, Stanley Chan, who also studied under Yip Man, does it differently and is less focused on power development, for example.