(6) It is often right in playing for an odd trick.
If your partner shows a weak game force him whether or not you are otherwise entitled to do it.”—Mathews.
With a weak trump hand force your partner:
“(1) When he has already shown a desire to be forced, or weakness in trumps.
“(2) When you have a cross ruff.
“(3) When you are playing a close game as for the odd trick, and often when one trick saves or wins the game or a point.
“(4) When great strength in trumps has been declared against you.”—Cavendish.
“Do not force your partner unless to make sure of the tricks required to save or win the game;
“Or, unless he has been already forced, and has not led a trump;
“Or, unless he has asked to be forced by leading from a single card, or two weak cards;