What the crib is
After the deal, each player keeps 4 of their 6 cards and throws the other 2 face down into the crib ACC r.1.5.c. The crib belongs to the dealer, who counts it last, after both hands. Once both players have thrown, nobody takes a card back ACC r.5.3.
The crib counts like a hand, with the starter as its fifth card, with one difference: a crib scores a flush only when all 4 crib cards and the starter are one suit, for 5 ACC r.1.7. A crib counts a flush only when the starter is the same suit too.
What to throw
Start from the four cards that score best together, then think about whose crib it is. When you deal, the crib is yours, so throw cards that help it: fives, pairs, and cards close in rank or that make 15. When your opponent deals, throw cards far apart in rank that give them little, and keep your fives.
Take 7 8 9 K 2 5. The 7 8 9 make a run and the 7 8 a fifteen, so you keep them either way.
Throw K 5. Together they already make 15 in your own crib.
Throw K 2. Far apart, they give your opponent little, and you keep your 5.
The discard calculator works this out for any six cards: every way to throw two, the hand averaged over every starter, and the crib valued for dealer or pone.