Dataminer Maksim "Gabe Follower" Poletayev spotted several lines in CS:GO Source 2 files, mentioning "tickless" system. He reported this on Twitter and tried to understand and explain how it operates.
"This might mean variable tick rate with a maximum cap", says Gabe Follower. What does it mean? According to Wikipedia, tickless systems operate like this:
A tickless kernel is an operating system kernel in which timer interrupts do not occur at regular intervals, but are only delivered as required.
Here's how Lorenzo0852 elaborates on this on CS:GO subreddit in the related topic. The user claims to have experience in video game development (although not network programming). Basically, the tickless system is when a server doesn't process the whole picture but only reacts to the changes, adjusting the events according to them.
Right now, it probably has an exact replica of the game scene in the server, and it's running and validating it every 1/tickrate seconds, so when you walk or shoot, it's not only verifying if your shooting is valid, but if every shot of every player is valid, if the movement of every player is valid, if all players are able to see each other or not, etc.
Similar systems that I've seen that could be considered "tick" or "tickless" mean that the server will only verify the action when it needs to be done. So if you move, it will then ask if that movement is valid, return it to you (the player), and then you'll see the actual movement. Right now, when you moved, you moved instantly and it was running background checks (comparing the result).
This would mean much faster servers as they don't really have to verify the whole scene at every single moment. And even if all players do similar actions at the same time, it would still need to verify much less than the whole scene.
To sum it up, maybe CS:GO players will have an even faster system than 128-tick servers. Valve was always against the transition, explaining that it would hurt users with weaker setups. Maybe this is the solution that removes the issue.
Fans worry that the change might affect utility in CS:GO. Currently, the servers with 64 and 128 tick rates have different lineups for the nades, as the server picks different timings of the jumps for calculations. But if to believe the above explanations, it shouldn't be a problem: whenever the pin release happens, it should trigger a consistent effect regardless of the server tick rate.
Counter-Strike 2 or CS:GO Source 2 is rumored to come out in March in the form of a beta test. Supposedly, the games will co-exist with each other for some time until the new version overwrites the previous.