As several obituaries to Charlie Watts have noted, his drumming was key to the sound of the Rolling Stones — he played "with a minimum of motion, often slightly behind the beat," which "gave the group's sound a barely perceptible but inimitable rhythmic drag," as this piece in the New York Times put it.
But Watts also had an interesting trick that was key to his sound: When he hit the snare drum, he'd typically not hit the hi-hat.
This has two side-effects: It slightly isolates the crack of the snare drum, so you can hear it a little more prominently. And because Watts wasn't hitting the hi-hat on that beat, he'd move his right hand upwards and out of the way, allowing his left hand more room to smack the snare even harder. So, two things that together gave his snare drum a unique hit.
If you want to learn to pay that rhythm, that YouTube video above offers a good lesson!
Watts's technique involved idiosyncratic use of the hi-hat, the sandwiched cymbals that rock drummers usually whomp with metronomic regularity. Watts tended to pull his right hand away on the upbeat, giving his left a clear path to the snare drum — lending the beat a strong but slightly off-kilter momentum.
Even Watts was not sure where he picked up that quirk. He may have gotten it from his friend Jim Keltner, one of rock's most well-traveled studio drummers. But the move became a Watts signature, and musicians marveled at his hi-hat choreography. "It'll give you a heart arrhythmia if you look at it," Richards wrote.
To Watts, it was just an efficient way to land a hard hit on the snare.
"I was never conscious I did it," he said in a 2018 video interview. "I think the reason I did it is to get the hand out of the way to do a bigger backbeat."