Now consider how these ideas can look when you’re playing defense. The two sides are fighting for control of the center; Black would like to get rid of one of White’s central pawns. He attacks the pawn on d4 twice (with his queen and knight), and it is defended just once (by White’s knight). So should he take it with Nxd4? It is important to visualize such sequences and consider what would be possible on the board as it would look afterwards. Picture 1. …Nxd4, 2. NxN; QxN. The implications of the resulting position are clear if you notice the kernel for a discovered attack that White has on the d-file all along. That kernel is reason for Black to act with great care in any operations on that file; taking the d-pawn is asking for trouble, because it creates a target for White’s queen after it is unmasked. In this case the bishop on d3 would end up able to give check with Bxb5, winning Black's queen on d4 next move. So Black dares not play Nxd4 in the first place after all.
Notice how those quick exchanges on d4 ended up leaving White with a new way to check Black's king and also with a great new target for his queen to take once it's unmasked. An earlier point repeats: when a contested pawn lies in the center, don't just ask who has more pressure against it. Ask what the board would look like after those pressures are spent.