My understanding is that this is allowed when a declarer has "technical" problems. So in a sense, the question is about what constitutes a legitimate technical problem.
If the declarer wins the first trick, he can pause, because there is the issue of what to lead next.
But suppose there is a singleton of the first led suit in the dummy, the play of which is forced. Suppose the declarer anticipates having a technical problem in the closed hand regarding whether to accept or refuse the first trick. Can he pause to study the whole hand before playing from dummy or does he need to play immediately from the dummy and wait until the trick arrives at his closed hand before pausing? That is to say, since he is playing both the dummy and hand, can he pause at the dummy based on a problem in hand?
Suppose dummy has a singleton, East plays the ace, declarer has Kx (or Qx), and wants to make a decision whether or not to false card with the honor. Is this an allowable technical problem for a pause either at dummy or in hand?
If the opponents retain the lead, must the declarer wait until he has a technical problem regarding an opposing lead to pause and study the hand?
And finally, can the two sides mutually agree that the declarer will not play from dummy for at least say, five seconds, so that BOTH sides will have a chance to study the board?