SIG p226 blowback function

I own two SIG p226 CO2 blowback air pistols. I read an internet article explaining how to bore out the gas valve for more power. One person commented that he had also disabled his slide blowback to conserve CO2.

I have done other modifications to my guns but I cannot figure out how the blowback slide works. There appears to not be a separate gas port for the slide anywhere.

Can someone explain to me how the CO2 operates the slide? And how to disable it?