The practice of taking a photograph at the recommended speed, then taking two additional photographs, one at one f-stop higher and one at one f-stop lower. This technique helps guard against problems seeing details in dark areas and light areas (blown out) by recording the same scene at different light levels (so, something too dark to see in the middle photograph may be visible at a different f-stop).

Source:

Redsicker, David R. and O’Connor, John J. Practical Fire & Arson Investigation (Second Edition). CRC Press. 1997.