This convention is used for distributions like 4-4-4-1 and 4-4-5-0 to set the contract and express slam interest
Once this distribution is established either through the diamond takeout opening or potentially the one of the responses to 2♣ Opener
You may only break a 4♦ transfer if you opened 1♣ and you have an exceptional hand to avoid risk of someone passing the 4♥ and you think slam is possible
4♣ - is a relay to 4♦ with slam interest, next bid is to set the suit and show interested in slam, split your range and go when top
Can use 3014 for asking if Kickback isn't available after this
4♦ - is a relay to 4♥ and forcing sign off in game including passing the 4♥, next bid is to set the suit and inform the person to pass the next bid
4♥ - is Kickback RKC for the lowest suit asking for key cards. This is usually ♣ but could be ♦ if you know club is the short suit
4♠ - is a Kickback RKC for the second lowest suit, this is usually ♥ but could be ♦ if clubs or hearts is the singleton or void
4NT - is Kickback RKC for the highest suit, this is usually ♠ but could be ♥ if spades is the singleton
Link to 2 Over 1
Link to Montreal Relay
Link to Precision