Torsion Cannons are very interesting weapons uniquely tied to the Adeptus Mechanicus. A sort of force-field weapon, akin to conventional Grav-Weaponry or the Ork's Tractor Weapons, but in its own unique twist.
You see, Torsion Cannons are mounted on Kataphron Battle Servitors, the tri-sectional barrel of the torsion cannon is amongst the holiest of Mars’ weapon technologies. When energised, it sends out three synchronous gaol-fields that hold sections of the target in place. As the sections of the cannon’s barrel turn counter to one another, the matter grasped in the fields is mercilessly twisted, subjected to an impossible torque that rips, buckles and ruins alien monstrosities and enemy war engines alike. Hence the name, torsion cannon.
Crunchwise in 9th Edition, Torsion Cannons are slightly cheaper a Multi-melta that lacks the melta rule, single shot on a BS4+ Breacher. Unreliable vs vehicles, overcosted vs infantry, the Heavy Arc Rifle will serve you better.