Refers to taking a substantial enough first gulp from a pint of Guinness that the level of the remaining beer in the glass is directly through the middle of the "G" in the word Guinness, which is printed on the glass.
I can split the "G" when drinking a 16oz American pint, but have yet to do it with a 20oz Imperial pint.
On your first try of drinking a Pint of Guinness you drink enough to where the black of the pint is directly in line with the line in the middle of the G