Phasing will make the commons of two or more transformrs truely common. They can be combined into one main common post among multiple transformers.
Some outputs would not use that same common however. Those power taps should (still) be kept 100% isolated from any other posts, i.e., they still function as usual.
Doing careful voltage checks there are workarounds for sometimes using these odd acc. taps with leg on 'main comm', but positions become more crucial to do it, and you might loose another tap or throttle to use some taps as a "main common".
Out of phase can double voltage or cancel itself out by messing with the "timing" of opposing waves. Phasing syncs the taps we wish to use. The lionel video kinda assumes what taps you wish to use, but the theory can really be applied elsewhere. The video gives the important base theory; it applies everywhere. Even knocking down 440 to 220 to 110v, do it out of phase and you have issues.
