Quick stall is just effectively stalling through offensive pressure, constant pivoting rather than defensive invincibility + phazing to force switches and pile on hazard damage.
Basically something like what you do with Celebi/Arcanine, Flygon/Milotic, Slowbro or Mew/Zapdos setups or even Golurk/Stoutland. What they all got going for them is resists/bulk plus a ton of damage, can afford to run every 1-2 turns and you don't really have a choice against them but to switch or do non-fatal damage in return for being KO'd.
The big difference is you're not really aiming to stall or sweep but the sheer knowledge they can do so is what forces your opponent to make switches because its the sensible thing to do and you've effectively pulled off the same thing dedicated stall teams aim for. Lots of people do it already without recognizing it but quick stall is just to distinguish it from common defensive stall and its different enough to warrant it. It was a coined term being thrown around near end of Gen 4 and early Gen 5 and I like to use it.
Not as slim as we'd be led to believe, I see this kind of shit happen way too often to be as unlikely as you'd think. I've lost matches completely thanks to Froslass alone and 7+ turns of being screwed over by paralysis/misses or having a bloody Spinner D-Bonded. Hell as I post this I just lost two matches in a row simply to Froslass haxing its way 7+ turns through to full hazards and having zero opportunities to spin from being beaten down by high powered blizzards afterwards.
