This convention was instituted after chaos resulting from such a move in the notorious Frobisher v Fisher second-round match from the 1938 world championship. Following a perfectly innocent shunt by Frobisher, several of Fisher's well placed green tokens were dislodged. The game rapidly degenerated into shove ha'penny and was abandoned shortly afterwards (to be replayed the following day) when the police had to be called. A blonking manoeuvre can force other players to readjust their tokens as well leading to a 'round of blonking'.
[TCM]