Yeah...kinda weird, but he's doing it to help train my mind -- so I can be calm and relaxed no matter the pain...he's "the mind" afterall
i have such conflicting opinions on this man (i'll have to meet him one day to find out what he's really like) but he's very smart when he's not trying to sell something.
by keeping your face calm, it calms your mind aswell so you can challenge your focus on what you're doing and achieving and, in a bodybuilding sense, obviously that would mean targeting the intended muscle group and creating more forceful and focused contractions.
i learnt this from when i started competing in boxing. novices, especially hws get hyped up, angry and charge in all tense and i was no exception. on the night with the crowd and everything, there is too much adrenaline flowing. i was told by my amateur coach at the time (few years back now) to 'knock this guy out'. well, i could do it in sparring and i did want to be mike tyson so that's exactly what i tried to do.
cut a long story short, i lost the fight (lucky i didn't get ktfo myself), i was so exhausted that i didn't recover for hours after (a police officer at the fight said he saw corpses that looked better than me - i was blue from lack of oxygen

) and when i look back at the video, there's the fucking coach slapping me in the face and yelling at me in between rounds (as if i wasn't tight enough

). thing is, plan, defense, power, speed, stamina - focus went out the window and that's why i lost.
the reason i bring up fighting is because there are not many other sports where the flight or fight rush kicks in more, yet the best fighters ALWAYS stay calm. same with car racing, anything. staying calm under pressure is an extremely important lesson in sports/life and yes, it starts with the face.