Proprietary algorithms within the tstats software.
I install a 3 ton 2 stage 16 seer 2 years ago I use a Pro Vision 5000 tstat.3 stage heat and 2 stage cool. The tsat uses algonrithms .My home had a 4ton single stage and after doing a manual "J" the home only needs around 28000btu's senable and 4000 btu's laten. The only way I know that it changes is by the tstat internal relays bearly clicking. The light bill drop a lot and the humetity stays around 47%-51% the system runs most all day (98*-102*) when its HOT.The house fills very conforable .I say- size the unit by manual S,load by manual J and duct by manual D size the registers by manual T.I did a home in Ludowici Ga. 2.5 years ago and there light bill only once went over $100.00 on month and thats because in deep well was water log and runing al the time. He leaves his tstat set on 74* his house is 1540 sq. ft. 2 ton 2 stage . First stage is around 1 1/4 ton.(67%)
You've pretty much got the idea of how it works.
Each t-stat manufacturer has their own way of doing it & some are adjustable but basically
they use a number above the set point & a time the set point is not reached. Some actually calculate whether or not the temp has changed enough in a certain amount of time before allowing the 2nd stage to come on. Some White Rogers have an adjustable setting in the configuration mode that will let you bypass the time limit if the set point is x amount above the actual temperature. If you look real close at the setup & configuration in your manual you might find something about 2nd stage.
Thanks for all the replies, I just wish I knew more. If I do get a dual stage heat pump and my existing W-R doesn't give me some owner flexiability, I may need to look at other tstats, because this seems like a pretty important issue.
On the W-R, fast cycle is stated as 1.2 degrees, so if set point is 77 F, then the tstat should turn on 1st stage at 78.2 F. But temp at the tstat may continue to rise in the first few min. due to 1) hot air expelled from the ducting, 2) coils not completely cooled down yet and 3) heat gain exceeding 1st stage capacity. But 3) is the only valid reason to change to 2nd stage. So, if the tstat makes a decision too soon, 1) and 2) may lead to an unnecessary 2nd stage call up and if it waits too long, indoor temp make become uncomfortably high due to 3). Seems likely that the tstat would be making a decision at say 5 min. intervals.