That's a great question. Carnosine is formed when two aminos, Beta-Alanine and L-Histidine, combine. What Carnosine does is act as a buffering agent that reduces acidity inside muscle cells. Here's how it works. During exercise, as energy is broken down to fuel movement, hydrogen ions accumulate and lower the pH balance inside working muscles. This creates an acidic environment and creates "the burn" that precedes muscular failure.
Thus Carnosine buffers against…or soaks up…hydrogen ions. This minimizes the acidity within the muscle. When that occurs, one can effectively delay the burn and muscular failure, which results in increased work capacity – ie, the ability to get more reps.
