I've been working on a 2600 emulator (yes, I know about Stella but this was so I could do easier integration with other potential projects).
I have HMOVE and all it's oddities working as best as I can tell but I'm a bit unsure on RSYNC. Looking at the schematics it appears to just force the main counter into the reset state (after 3-6 clocks depending where H1/H2 timing is at).
So happening mid-scan line would potentially give another full set of pixel clocks to work with? i.e. triggering right at the end of HBLANK in theory would restart and give another 21-22 cpu clocks or so before painting occurs? The schematic looks like it's a normal reset of everything so it asserts resetting the HSYNC and disabling the beam, etc.
I read through some commented space invaders code and it's used twice there. Once right after WSYNC and another it looks like as I described above (trigger WSYNC, compute a few things and then RSYNC to recover clocks).
This isn't super well documented that I've found so until my Harmony cart arrives and I can write some test code I'm mostly stuck at interpreting 40 year old schematics