How do the turbo makers "take compression out?

[snowmanx]

I am reposting because I don't think the title was clear...

Stock M1000 runs 12.1:1 compression ratio. Sounds like Twisted/ovs take compression out to the tune of 10.1-10.5? How is this accomplished? So they cut the dome out?
Thanks

 

[snowmanx]

Twisted does not take any compression out of the head. retains stock C/R.

aaaa

 

[jijohans]

Some rework the head with bigger/deeper domes, some cut the squish down 4/10-6/10mm...