A declarer who tests the distribution of the outstanding cards in the unseen hands before committing himself to one line of play is making a ‘discovery play’.
For example:
West leads the ♠J against South’s 7NT. You have twelve top tricks and the thirteenth will come from a successful finesse against the ♣Q. By playing off the winners in spades, hearts and diamonds, you find that West started with a 6-3-3-1 distribution. You can cash the ♣K and confidently finesse against East on the second round of clubs.