[PantsOnFire]

This question has too many layers.

I'll just give a simple answer that applies mostly to NL. Marginal starting hands have low value preflop compared to big hands. This should be obvious. So if you go all-in with AA vs. J9s, you will win most of the time. However, if J9s calls a raise and sees a flop vs. AA, sometimes those preflop percentages change drastically in favour of J9s. In other words, preflop, everyone has a certain pot equity based on the strength of their opening hand. However, the three cards on the flop change all the hand equities. This is where the trouble begins.

There could be trouble for big hands that are now way behind or their could be trouble of little hands that now look a lot better but are second best.

Preflop hand selection and play is important, but in NL how the flop is played is even more important, especially to guy who are playing marginal hands.