The tau, also known as the gauss, is perhaps the most important weapon in AG. With it, you can gain speed and maintain it with bunnyhopping, as well as fire through walls. Primary fire quickly uses up the ammo; don't use it unless you're sure your opponent is low on health. Secondary fire is much more useful. You can charge up the shot, and fire it to launch yourself in the opposite direction (more effective when you jump right before firing).
The secondary fire can also shoot through some walls, depending on how thick the wall is. The thicker the wall, the less likely the tau is going to hit anyone behind it. This feature is useful for flushing out campers.