For you thereby take the best chance of preventing his making use of his trumps for bringing in a suit. If he refuses to take a force, keep on giving it to him.
For instance, if he passes your king (led from king, queen, &c.), and the king wins, continue the suit, and so on. Some players can never be brought to understand this; they do not like to see their winning cards trumped, and therefore frequently change their suit or even lead trumps when an adversary refuses to be forced.
It now hardly requires to be stated that it is bad play intentionally to force a weak adversary, and stillworse to lead a suit to which both adversaries renounce, as the weak will trump and the strong get rid of a losing card.
If you have numerical strength in trumps, you are justified in forcing your partner, relying on your own strength to disarm the opponents. But