Weld is one of the most used tools ingame. It takes two props at player defined locations and puts them together as one. As a result, picking up one of the objects with the physgun will cause both to rise. The props will react as one. Shooting one with the gravity gun will cause both to move as usual. Objects can be welded when they are not even touching. Ragdolls can also be welded to other ragdolls and/or props. Both ragdolls and props can be welded into the world (floor, ceiling, walls, ect.)
The weld tool as a variable called Force Break Limit. This defines the amount of force needed to break the weld between the objects (and/or world). GMod animators usually input between 1000-2500 on props welded to a ragdoll's hand(s) to simulate throwing. Such FBL values can also be used on props and ragdolls welded into the world for situations where a bullet, vehicle, explosion or another prop/ragdoll strikes the welded object to break the constraint and knock the object down. Despite 0 being the lowest value, the weld cannot be broken unless undone (default undo button: Z).