Breathing techniques AND exposure to cold.

From the article:

ii) During the training, subjects voluntarily exposed themselves to cold in several ways: standing in the snow barefoot for up to 30 min and lying bare chested in the snow for 20 min; daily dipping/swimming in ice-cold water (0–1 °C) for up to several minutes (including complete submersions); and hiking up a snowy mountain (elevation: 1,590 m) bare chested, wearing nothing but shorts and shoes at temperatures ranging from −5 to −12 °C (wind chill: −12 to −27 °C).