L-Leucine is the free-form version of the branched-chain amino acid leucine. Taking it alone spikes blood levels faster than getting it from food or a BCCA blend. That rapid rise helps turn on muscle protein synthesis.
Many people dose it around workouts. Timing it before, during, or after exercise can speed muscle repair and cut down next-day soreness. Some rely on it for steady energy during long sessions.
High doses can upset the stomach. Start low and increase slowly to see how you handle it.