Nitric oxide is a molecule responsible for vasodilation which allows more nutrients & oxygen delivery to your muscles at a faster rate. This results in better pumps, more endurance & an overall better cardiovascular performance.
L-Citrulline works by way of converting into arginine and has been known to increase nitric oxide & plasma arginine levels at a higher rate than L-Arginine itself due to the first pass effect where most arginine gets metabolized in the stomach prior to reaching the bloodstream.