What's the logic behind this chain length estimation method? I know of several methods of sizing a chain for a bike:


As many links as the previous chain.
Small-small. The RD should be barely expanded, the chain nearly rubbing against itself roller-cogs. The logic here is to be able to have the longest possible chain, that still allows shifting into any gear combination.
Vertical rollers. Shift big front small rear. The rollers on the RD should be in a vertical line, below the cogs. In this position the RD is in a good working point, because it's spring is both tensioned (and thus stiff against vibrations) and not over-elongated (and suffering accelerated wear). This is the medium working point.
Big-big without the derailleur. Without threading the chain through the RD, measure big-big and add 2 links.


My question is what is the motivation of the last method. To me, not threading the chain through the RD just doesn't make any sense. What physical quantity is being measured by shifting to big-big and why are 2 links being added?

As a side note, I recently tested all of those, and no two returned the same result.
Big-big and through the dérailleur is the most chain you'll ever need as you can't shift to any gear which takes more chain than this.

You will then need to add some links to take into account the extra chain required to go through the dérailleur.  2 links is the least amount you can add and when the rear mech is as stretched out as it would near maximum extension, the chain doesn't need to be much longer to accommodate this.

Theoretically you could put the chain on big-big and through the dérailleur and try and gauge how long the chain should be for the mech to be almost fully extended but not quite, but by doing big-big plus 2, you should get a similar result.

On your side note that each method gives different results, this could be because there is no correct answer.  The big-big plus two gives you a safe chain length in that if you were to shift into big-big there would be sufficient chain to do this.  If you don't cross-chain, you could get away with a shorter chain length, but risk damaging something if you were to shift into big-big.