This convention applies after 1 Diamond respond to 1 Club openning. Opener does not declare his/her 4 cards major even if he/she has and declare 1NT. Because opener knows that either responder does not have a 4 cards major or if he/she has, will declare at the second tour. In this way, opener wouldn't give information about his/her 4 cards major to the opponents, if responder does not hold a 4 cards major. Lets' see the application in a tableau.
Responder does not have a 4 cards major with 6-9 HCP
Responder holds 5 cards Diamonds and 4 cards Spades/ Hearts with 11+ HCP. Now opener may support the responder or declare his/her 4 cards major.
Using this convention, if opener declares 1 Heart or Spade after 1 Diamond respond, it means he/she holds 5 cards Clubs and 4 cards Hearts/Spades.