What are the preconditions for a trump promotion in bridge?
The Bridge Encyclopedia identifies 4 situations in which trump promotions ("the creation of trump tricks by forcing the premature use of trump cards by the opposition").


Uppercuts - ruffing high to force opponent to overruff, thus promoting a trump trick in partner's hand.
Establishing long tricks in trumps by forcing opponent to ruff - as in a forcing defense.
Coup en passant to create a positional trump trick. Example:

      K
      -
      -
      2
A           Q
-           -
-           A
A           -

      J
      -
      2
      -


With spades as trumps and the lead in the south, if declarer (south) leads a trump, they lose both tricks. But if they lead a diamond, the spade King will be scored en passant - either west will trump and declarer will discard from dummy, or west will discard and declarer will ruff with the king.
A positional overruff threat. For example, if south holds KQx in trumps and west holds AJ, playing on trumps will lead to west taking a single trump trick. If instead east plays on a side suit in which south and west are both void, south must ruff high to prevent an overruff with the J, but this establishes the J as a second trump trick (as long as west does not overruff with the ace!).