160 spins precess at slightly different rates, so their summed magnetisation fades. At time τ a π-pulse mirrors every phase, and the fast spins are now behind. At 2τ they all line up again.
Tap to fire a π-pulse yourself. The same ?seed= always gives the same run.
LITHahn’s spin echo (1950). |M| must stay inside [0, 1] and every phase must stay finite, or the page throws.
AMBERSpins are drawn at half-angle, so one turn of the dial is 720° of phase: the site’s spinor clock.