Citrulline Malate is derived from the non-essential alpha amino acid citrulline. Citrulline is not used for muscle synthesis as many of the other amino acids are, instead it’s involved in metabolic byproduct removal (such as nitrogen and hydrogen) and energy metabolism i.e. energy production.
Perhaps one of main benefits of citrulline malate, certainly in the sport supplement industry, is its ability to induce vasodilation (widening of the arteries). This is why you might find citrulline malate on the ingredients list or proprietary blends of many pre-workout supplements. The widening of the blood vessels means more blood reaches the muscles, brain and other major organs enabling you to train harder for longer. Also its capacity to buffer byproducts such as hydrogen means your blood stays in an alkaline state for longer, enabling you to train for longer.
Citrulline can be found in food sources such as garlic and onions, and in particular watermelon. Watermelon's citrulline content is readily converted to arginine, which incidentally is also converted to nitric oxide...the key component of many top pre-workout supplements. Oh and I forgot to mention, you'll also find it in supplement form too!
Llewellyn, W. (2009). Sport Supplement Reference Guide. Citrulline Malate. FL: Molecular Nutrition LLC.