Yes, I am a source for this information :)
Here are some pictures I took of the escape system while standing on the launch pad tower during the STS-124 Terminal Countdown Demonstration Test.
The idea was that the crew would egress the Orbiter and run down a walkway to this platform. Then jump into the baskets, 2 people per basket. You can see the sign LEVER UNDER SIGN RELEASES BASKET
Pull the lever and the basket whizzes down a wire rope. Draglines drag in this pile of sand which serves to slow it down. Once it stops, the crew jumps out.
Then they get into the "tank" (really an M113 armored personnel carrier) and drive away.
Source: personal photos
It was never used and NASA management was afraid to have anyone actually test it out. But astronaut Charles Bolden (later the NASA administrator) insisted on testing it and did, becoming the first person to ever ride it.
It would have had to be a very slow emergency for the crew to be able to unstrap, climb out, open the hatch, run to the baskets, slide down, and get into the M113 before whatever was happening...happened
You can read more about how the "tank" was also used by rescue personnel here: Ground personnel proximity to space shuttle at launch?
Finally, here is a video of the last time the baskets were released.