Discovered attack
A discovered attack is released when one of your pieces moves out of the way of your bishop, rook or queen and suddenly opens its line of fire. If the revealed attack hits the king, it becomes a discovered check — and the moving piece may then go almost anywhere, often capturing material with tempo. The double threat of the new square plus the opened line is what makes discovery so hard to meet.
When the knight leaves f6, the bishop on b2 fires down the long diagonal at h8 — a discovered check.