S-Trig, short for Switch-Trig is an electrical short to GND. V-Trigs, as used elsewhere in the moog and Boog55 are the voltage-step type you find in EuroRack. More commonly called gates.
Patchcords can be bought or made to convert from V-Trig to S-trig. It's only a single transistor and resistor in one end of the cord. If you can solder, you can make them yourself. If you don't solder, they can be found already made.
So you don't have to use a 961. However it does do more than simply convert trigs, so you might want one anyways
One nice thing about S-Trigs is how easily they can be combined. In parallel, they natively function as a logical OR, which is something you need a special cord or module for in V-trig (gate) based systems.
Another nice thing about S-Trigs is how easily you can create them on the fly. Simply touch the free end of a patchcord that's already in the S-Trig input to any other system GND. Like a jack nut, for example