It's not VRR.
Dynamic Action Sync is LG's equivalent of BenQ's "Instant Mode".
Also, it is not available on every single LG monitor, so make sure it's available on your model.
To enable the ability to turn on Dynamic Action Sync, use full screen mode (no GPU scaling) and maximum refresh rate.
I think it's basically the use of a realtime synchronous scanout -- realtime refreshing the panel off the cable. This is an intrisinic feature of eSports-league monitors to have sub-refresh latency, where cable scanout becomes symmetric with panel scanout.
Instead of a monitor buffering the refresh cycle of the cable before refreshing the panel, the panel is refreshed in essentially realtime while the pixels are coming off the cable (with only rolling line-buffered processing for processing/etc).
This is simply the nature of serializing a two-dimensional image (a refresh cycle) over a one-dimensional cable, so pixels are transmitted one pixel at a time, left-to-right, top-to-bottom, and it takes time for pixels to transmit over the video cable (DisplayPort, HDMI, etc).
To reduce lag, a panel can be sequentially refreshed with the pixels essentially right off the cable. eSports gaming monitors with "Instant Mode" processing or things like this ("Dynamic Action Sync") is essentially the ability of a monitor to do sub-refresh-cycle processing this way by streaming the pixels straight off the cable almost straight onto the LCD panel with minimum technologically possible processing latency.
See
High Speed Videos of LCD Scanout.
Monitors with "instant" processing ability, will have a pixel transmission sequence that looks like this on the cable AND on the panel, with no scan conversion or full pre-framebuffering step between the cable and the panel.
Reducing most of the signal latency mostly down to just pixel response latency (And any cable latency overheads, e.g. micropacketization/codecs/etc), even achieving input lag of less than a refresh cycle for bottom screen edge.