DAR follows the same idea as the polarization index but over a much shorter window: it divides the insulation resistance measured at one minute by the value at 30 seconds. The whole test is over in 60 seconds instead of 10 minutes.
It is used when there is no time for the long test, when a first screening is wanted before deciding whether to continue, or on small equipment where the absorption curve settles early. The reading serves the same purpose: spotting moisture and contamination without temperature skewing the result.
The usual interpretation puts below 1.25 as unsatisfactory, 1.25 to 1.6 as acceptable and above 1.6 as good. Being a shorter window, DAR is less sensitive than the polarization index, so a sound DAR does not remove the need for the long test when there is a well-founded suspicion.