"Double tap" means taking two quick shots, one just after another, at one target. In combat shooting this technique is used to provide a maximum certainty that your target is neutralised. It's very important when using the 9mm cal. which doesn't have as much stopping power as i.e. .45 ACP. In practical shooting double tap is used for realism and to maximize the probability of hitting the target. Because of great dynamics this technique requires a very good control over the recoil.