This is such a cool technique! Gravity lensing makes two images of the same supernova, but one takes longer to reach us. The first image tells us what to look for in hindsight, so observers were ready to study the supernova in detail in the second image.
A very rough analogy would be using an echo: the first time you hear the sound you know to turn on your audio recorder and then a split second later you record the sound of the echo in detail. But that analogy is weak because this is bent light beams, not bounced signal like an echo.
Originally shared by Brian Koberlein
Astronomers love this one simple trick!