Dear all,
Every then and when I get questions here in the forum or over at flightsim.to that can essentially summarised as “what does Sky Dolly actually record?”, or more generally: “How does recording / replay work?”
But to first address another elephant in the room: “Yes, Sky Dolly is still in the makings” Development has slowed down somewhat in the last three months, because I changed my job - life happens 
But as I write those lines I have been adding some more code to the upcoming v0.17.
Don’t expect anything big this time around, as this will be more of a “refinment release”, polishing existing functionality and smaller issues and ideas that have been reported/suggested by community members - thank you all for your support!
Now for the questions: the latest specific question that I have received was:
“Does Sky Dolly capture and replay smoke from tires upon landing?”
Similar questions that I have received in the past were related to audio, whether Sky Dolly would record that, too.
And the answer to all those questions is a simple: “No.”
So how does it work then? Sky Dolly (and other flight recording software) records a selected set of what is known as “simulation variables”: they represent the state the simulation and the aircraft is in at any moment in (simulation) time. Those variables are part of the official SimConnect API that has been introduced back in the days with Flight Simulator X and continues to be supported by MSFS today.
The “simulation variables” represent all sorts of states, for instance:
- Aircraft position and attitude (= as a result of the user input and simulated physics)
- Control input: yoke, rudder and the resulitng ailerons postions, flaps, spoilers, …
- Animation states: cockpit open/closed, …
- Environment: temperature, wind directions, time of day, …
Not all variables can be modified, as they are a result of the calculated physics, based on other simulation variables. And to make things a bit more complicated: not all aircraft properly report (support) those “simulation variables”, let alone properly react to changes in them (but that’s another topic, for another time ;)).
So what does Sky Dolly record then? A selected set of those variables that have “the (biggest) impact on the visual representation of the recorded flight”. So for sure the exact position and attitude, but also “visual clues” such as flap/brakes/ailerons/… settings (“pilot input”). Also light switches and other “visually relevant cockpit switches” are recorded.
The exact set of those “simulation variables” can be seen in Sky Dolly, by opening the “Simulation Variables” (sic!) dialog (e.g. by pressing key V):
The variables are grouped: more or less in the same groups that Sky Dolly uses to communicate with the flight simulator. “Why is that so?” you may ask: because that’s a clever way to minimise both the required disk usage and CPU usage while replaying those variables to MSFS. Only those values that are actually changing are recorded (= less disk usage) and sent back to MSFS during replay (= less CPU usage).
For instance the gears are typically only raised and lowered once during a flight. So it would be a waste of disk space/CPU to always send a “gears up” value to MSFS. (And yet this all works magically when you seek back and forth at any time during replay ;)).
So to narrow down on the specific question, whether “gear smoke” or “audio effects” are recorded: again, no. They are the result of the above simulation variables being replayed to MSFS - or not. While most of the physics keeps running during replay (instead of you giving control input it is now Sky Dolly) every physics simulation that would affect the aircraft position and attitude is disabled during replay. Otherwise the MSFS physics would constantly “fight” the injected aircraft position and attitude, for every “simulated frame” - and the result would be a “jittery user aircraft” (there is another known issue related to “jittery AI aircraft”, aka “formation flying”: but that is yet another topic and not directly related to “physics fighting recorded position”).
Because of “position / attitude” physics are disabled - by design - it could be that some “visual effects” are not triggered (simluated) as they usually would. Also keep in mind that since we are only recording a subset of all possible simulation variables the simulation during replay is not 100% the same as when you recorded the flight. So slight simulation alterations may cause some effects to be ignored (or vice versa, you keep hearing warning signals that weren’t heard during recording).
I am still looking into recording more “simulation variables” that could have a “significant (visual) effect” on the replay, for instance I am looking into payload weights and fuel that incluences the display of external ordinance / tanks on some aircraft, such as the F-16. Also, I intend to completely re-engineer the recording of the engines: as you know MSFS now supports a total of 16 engines.
And to conclude: Sky Dolly also records some “read-only” simulation variables, but those are “for your information only”, as can be seen in the “Flight” (press key F) dialog:
I hope that answers some of the questions 
Happy flying!
P.S. Pro tip: hover the mouse over any of the above simulation variable text boxes and you will see the exact “simulation variable” name, as used via SimConnect and documented here:
https://docs.flightsimulator.com/html/Programming_Tools/SimVars/Simulation_Variables.htm
And if you want to experiment yourself, there is a simple “Simvar Watcher” example application that comes with the MSFS SDK (software development kit) examples, downloadable from within MSFS with enabled “developer mode”
Here is an older post illustrating the use of the “Simvar Watcher”: