The Piper Aztec (Lycoming IO-540 power) was quite prone to icing/clogging of the air filters, especially when flying in conditions of wet snow.
Normal air inflow was through forward-facing scoops beneath the nacelles, then upward through the air filter and into the induction manifold.
Snow/ice could accumulate on the outside of the filter and block the air supply.
The alternate air mechanism was a simple hinged door in the middle of the canister housing the air filter. Selection of alternate air would change the air source from the forward-facing scoop to the inside of the nacelle.
Pulling the alternate air knobs meant losing the ram effect of the intake scoops and a typical drop of a couple inches of MP. Not exactly helpful when the airframe was also accumulating ice -- but it was better than a complete loss of power.
Perhaps you'll have better luck than I in finding an Operating Manual for the old Aztec on line.
IIRC, the guidance spoke specifically to the conditions of heavy, wet snow.