If you want to twist two centers by 90 degrees each, there is a simple [3,1] commutator:
I'm having trouble figuring out what exactly that means. Could you write it out in normal notation?
In Gelatinbrain notation R&2 is the slice between L and R turned clockwise as R turns.
R'&2 is the same slice turned anti clockwise.
Correspondingly U&2 is the inner slice below U.
In former WCA notation (2010) R'&2 is equal to M (M is the slice under L turned into the same direction as L and
U'&2 = E (E is in WCA notation the slice above D turned as D turns.
So this [[R'&2:U'&2],U] translates to [[M:D],U] which is the commutator / conjugate notation of
the eight moves M D M' U M D' M' U'
If you use the former WCA notation for Big Cubes slices you would write r' u' r U r' u r U'.
EDIT: In WCA notation 2013 it would become Rw' R Uw' U Rw R' U Rw' R Uw U' Rw R' U'
If I use brackets for slice moves it becomes (Rw' R) (Uw' U) (Rw R)' U (Rw' R) (Uw U') (Rw R') U'
When I wanted to respond to BN's question yesterday, I wanted to quote the WCA notation. I found that simple slice moves do no longer exist and wrote about this here
Honestly, I hope that WCA will change this back!!!