Counterweights is a technology in Age of Empires II: Definitive Edition - Dynasties of India that is unique to the Saracens and can be researched at the Castle. Once researched, it grants Trebuchets and the Mangonel line +15% attack.
Saracens have fully upgraded Siege Onagers and Trebuchets. As a result, with this technology, their Trebuchet can take down buildings faster, and their Onagers will be more lethal to units. Due to friendly fire, the Onager should be more cautiously used when this technology researched, as it will also hurt a lot of allied units.
Counterweights is also significant in Trebuchet battles because it enables Saracen Trebuchets to take out an enemy's Trebuchet in two hits, as opposed to the normal three hits (repairs notwithstanding). It also allows their Siege Onagers to take down various units such as Hussars, Champions, or Bombard Cannons in one shot, even if only a few of the projectiles connect, whereas other civilizations would leave those units severely damaged (but alive) unless a direct hit is scored.