When one's partner has either led or has called for trumps, then the doubtful card should be trumped without hesitation, and the best trump led to the partner's call or lead of trumps.
One very common and oft-repeated error of the bad player is to refuse to trump a winning card merely because he holds four trumps with one honour. He will refuse to trump more than once, and imagines he is playing a strong winning game by discarding one or more worthless cards of a short suit, which he believes he will be able to trump when this suit is led.
With a hand of trumps not sufficiently strong to make certain of extracting all the trumps and remaining with the lead, it is advisable to consider how many tricks are likely to be won by the trumps in one's own hand. For example, the trumps held are queen, nine, four, and two. It is not likely that more than one trump will make a trick, and possibly not one. We have the chance of trumping a doubtful card, and we refuse to make even one of our trumps, and throw away a certain trick if our partner does not hold the winning card of that suit. If our partner does hold the winning card of the suit, he may not be obliged to play it on our trump; and it is no severe loss to make one trump out of four, even if the partner does hold the winning card.
It cannot be too strongly impressed on the young player that the great object of Whist is to win tricks, and to refuse to win a trick when you can do so is to accept a dangerous responsibility.
Underplay.
One of the worst forms of bad play is to repeatedly change the suit, and thus, by continuing to make your partner third player, to incur the risk of sacrificing the best cards of each suit that are in his hand. Instead, therefore, of leading a fresh suit in which you may be very weak, it is frequently safer to return the adversaries' lead, especially if it is evident that you can lead through the strong hand up to the weak.
In order to take full advantage of this lead, what is termed "underplay" may be attempted. As an example of underplay, the following is given. A, original leader, leads the two of spades, thus indicating
most probably a four suit; A, it is evident by this lead, does not hold king, queen of the suit. Y, the second player, plays the three of spades; B, third player, plays the nine; and Z, fourth player, holding ace, ten, and four of spades, wins the trick with the ten.
Z, having no court card in the three remaining suits, fears to lead any one of these. Z knows that he possesses the ace of spades, but this fact is not known to A. Z then underplays by leading back the four of spades. A, who holds king, knave, eight, plays knave second hand; Y wins with queen, and now knows that his partner holds the ace, for had A held the ace he would have played it second in hand. Y, now having the lead, can show his strong suit, and may fairly assume that his partner has no good suit, because, had he been strong in any suit, he would have led a card of it, instead of at once resorting to underplay.