I'm still a little confused as to how these formations work.
Do you just pick a detachment and give it a bunch of extra rules for free?
You can upgrade a detachment for 1cp. This gives certain units in that detachment an extra keyword. For example, all Leman Russes in an Emperor's Fist detachment gain the Emperor's Fist keyword. For all of the ones I have read, these keywords don't do anything by themselves. However, almost all of the extra stuff only works on these keywords. For example, you can pick a warlord trait for Emperor's Fist Leman Russes that only affects Emperors Fist Leman Russes within 6", or a stratagem you can only use on that keyword.
There is an additional thing. A new stratagem that let's you give a detachment's warlord trait to a unit in that detachment for 1cp, which you can do in addition to your normal warlord trait. So, using Emperor's Fist as an example, you can give the aura to a tank commander with the keyword, as long as he is not your warlord. A way to get these nice new, albeit limited use, traits without sacrificing your normal trait.