In the most common type of foundation brake, the S-cam drum brake, what happens when the driver presses the brake pedal?

AAMVA CDL Manual, Section 5.1.8 (Foundation Brakes - S-cam Brakes), p. 5-2; 49 CFR 383.111(b)(1)

S-cam brakes are the most common heavy-vehicle foundation brake, so understanding how air pressure converts to mechanical braking force at the wheel helps explain pre-trip checks like slack adjuster travel and why worn or misadjusted S-cam components reduce stopping power.