I had to look in case I was missing some cultural reference to "Pumping Cyanide" (Perhaps some counter-culture metal band?) and all I found was this:
The seat the inmate sits on has a bucket of acid under it. @ execution time sodium cyanide is dropped into the acid. That’s where it is introduced. Vent the chamber afterward, probably causes global warming. Your statement of pumping the chamber full just made me laugh for some reason.