It's an Angle-of-Attack based display; it basically has a green (target) range of AOA that you are supposed to fly in, with the low and high ends being red "no go" areas representing overspeed (low AOA) or stall (high AOA).
Fairly simple in concept - though of course it requires the AOA sensors be operational, and depending on the nature of the airspeed problem that might be in doubt (ADC hardware failure would be a good case for using BUSS, but something affecting the outside environment, such as extreme icing (SLD) or hail (physical gamage to probes and sensors) could disable BUSS as well as conventional airspeed)
Still, it's a fairly good solution in that it uses existing installed equipment, and has some significant dissimilarities to enhance the chance of it being available.