On a Trane, When you use a single stage t-stat, the W1 and W2 will need to be jumper together..
So when there is a call of heat, the board sees power at both lead right away, then the timer kicks in to hold the 2nd stage back to X amount of minutes.
(Dip switches can be set for .5, 5, 10, 15 min delay)
Now if you use a two stage t-stat, jumper is not used, and when the t-stat calls for heat, the W2 isn't powered up right a way.. There is a sec delay on contact.
so, either the units shuts down before 2nd stage time runs out, or the dip switch is set up at 30 sec delay, and pretty much fires up in high stage right away.
By a configuration DIP switch or jumper elsewhere on the board. The idea of sensing a jumper across W1 and W2 is fiendishly clever, and I suppose the Trane board probably does it by tying one of them internally high or low through an impedance and looking for matching voltage on the other one at times when the stat is not calling for heat. Saves them the extra DIP switch or jumper, and quick to set up in the field, and even sounds 'natural' once you know about it (as long as you're not thinking about how the board must be built to make it work).
If you don't jumper them, how is the unit going to know whether it's a single stage or two stage stat?
But then, to anyone who isn't familiar with that particular clever design, it looks a whole lot like somebody wired in permanent high fire. Maybe it is possible for a design to be too clever. (I might also wonder, if anybody makes a 2-stage stat that doesn't use totally separate dry contacts, if such a stat could fool the Trane board into thinking it's jumpered. Maybe nobody does and so it's a non-issue. Sometimes the K.I.S.S. principle is a good one.)
When you use the disadvantage of a timer you rob yourself or your customer of efficiency,comfort with temperature,&comfort with noise levels.when hooked to time out for second stage it will needlessly bring second stage heat in"running high fire all the time"!It should only run in high fire when temperatures are nearing design point.
Take your time & do it right!