When Sheldon "checkmates" the IRS agent, the agent knocks over his king. Knocking one's king over is a way resigning, it is not done when one has been beaten.
This episode is set around 1990, but Meemaw and Dale are drinking an oral solution, either Golyltely or Nulytely, to flush their bowels prior to colonoscopies. Oral lavage solutions (PEG 3350) were not approved for use until 2006, 16 years later. Bowel prep in 1990 would consist of fasting, laxatives, and enemas.