The declaration of two Hearts or two Royals is practically a command to the partner not to alter the call. It indicates at least six sure tricks, probably more, and a valuable honor count, in the Declarer's hand, provided the suit named be the Trump. The Third Hand should only change such a declaration when convinced beyond reasonable doubt that his holding is so unusual that he is warranted in assuming the responsibility of countermanding the order that has issued.
Weakness in the Trump and strength in some other suit is far from being a sufficient justification, as the chances are that the Dealer is weak in the suit of the Third Hand, and called "two" mainly for the purpose of keeping it from being named. To overbid two Royals or Hearts with three Diamonds or Clubs is obviously absurd, unless holding five honors and such other strength that game is assured.
To overbid two Hearts with two Royals, or two Royals with three Hearts, is almost tantamount to saying, "Partner, I know you are trying to shut out this declaration, but I am strong enough to insist upon it." Such action is only justified by 64 or 72 honors, and a sure game.
To overbid two Hearts or two Royals with two No-trumps, as a rule, means 100 Aces. High-card strength assures the game in the partner's call with probably a big honor score; only the premium of 100 makes the change advisable.
With strength, in the case under consideration, the Third Hand should advance his partner's call with much greater confidence than if it were an ordinary bid of one. He should not worry even if absolutely void of Trumps; in that suit his partner has announced great length as well as commanding cards; Aces and Kings of the other suits are what the Declarer wishes to find in his hand, and with them he should bid fearlessly.
The same line of comment applies with even greater force to the action of the Third Hand when the Dealer has bid three Royals or three Hearts.
WHEN TO OVERBID A PARTNER'S NO-TRUMP
When the Dealer bids one No-trump and the Third Hand holds five or more of any suit, one of the most disputed questions of Auction presents itself.
The conservative player believes that with five Hearts or Spades, inasmuch as but one more trick is required to secure game, it is safer to bid two Hearts or Royals, except, of course, when the Third Hand, in addition to a five-card suit, has the three remaining suits stopped. The theory is that if the combined hands are very strong, the winning of the game is absolutely assured with the suit in question the Trump, but may possibly be lost in the No-trump by the adversaries running a long suit. The chance of a hostile suit being established is unquestionably worthy of the consideration of the Third Hand whenever, with great strength in Hearts or Spades, he allows his partner's No-trump to stand. Five adverse tricks prevent a game. In the majority of cases, the leader opens a five-card suit. When it is not stopped, the game is saved by the adversaries before the powerful No-trump hand can get in; if it be stopped but once, the game is still in grave danger unless the Declarer take nine tricks before losing the lead.
With a Heart or Royal declaration the adversaries are not apt to take more than two tricks in their long suit, which, at No-trumps, may produce four or five (in rare cases six), and yet the Trump bid requires only one more trick for game.