I wonder if the term "impulse" here has any connection to the various impulse commands available in the source engine. I remember using "impulse 101" and causing havok in the opening plaza area. Spawning zombies on the roofs, sending them after the combine, etc.
In quake, the impulse commands were used mostly to switch weapons[1]. I'm not really sure about the naming though, why choose the word "impulse".
[0]: https://en.wikipedia.org/wiki/GoldSrc.
[1]: https://github.com/id-Software/Quake/blob/0023db327bc1db0006...
The impulse command sends a command to the server instantly, rather than in the usual UserCmd sent to the server 30 times per second. They are used mostly for debugging, just assign #90 to your debugging function, then poke it from the console while the game is running. No need to change and potentially break the network code.