What is actually happening. The 90° pulse tips every spin into the plane, all pointing the same way — that is the tall signal at the start. They sit in slightly different magnetic fields, so they precess at slightly different rates and fan out, and the signal dies. That decay is not the spins losing energy, it is them losing agreement, and it can be undone: a 180° pulse flips the fan over, so the fast ones now have to catch up from behind. They all meet again at exactly the same interval later, and the signal comes back — the echo.
Your job. Fire the 180° at any moment you like. The echo will arrive at twice that delay, so the timing you choose sets where it lands — but every shot has a hidden T2, a real decay that no pulse can undo. Wait too long and the echo is gone before you refocus it; fire too early and you never let it fan out enough to be worth recovering. Somewhere in between is the best echo this shot can give you, and that is what you are hunting.