From page 10-11 of [1]:

> A shared resource provides a way to build a real counting thread with a negligible overhead compared to a message passing approach. This already raised concerns with respect to the creation of a high-resolution clock [19]. In this method, one worker continuously increments the value of the buffer without checking for any events on the event queue. The main thread simply reads the current value from the shared buffer and uses it as a high-resolution timestamp.

[1] https://gruss.cc/files/fantastictimers.pdf

My understanding is: Have a web workers continuously increase a number, and save it to the SharedArrayBuffer. As long as the web worker is running, you have a steadily (enough) increasing timer that can be accessed by any other thread. It's not as precise, so relying on it makes things harder, but still usable.

By running a worker thread in a counting loop.