How does EMAS reduce damage in case of an aircraft overrun?
There is a link in the FAA tweet you link to, you should probably read it. 


  EMASMAX® is the latest, most durable version of ESCO’s EMAS, developed with and technically accepted by the FAA. EMASMAX arrestor beds are composed of blocks of lightweight, crushable cellular cement material designed to safely stop airplanes that overshoot runways.
  - FAA Fact Sheet – Engineered Material Arresting System (EMAS)


So yes it's like gravel runoff areas, where the crushable material slows down the aircraft by providing resistance through the landing gear. The difference being this is a bit more engineered so you don't get bits flying up into the engines etc. like gravel would.

As for specs, the same link claims it can stop an aircraft going 80 mph but doesn't state a size of aircraft. 


  A standard EMAS installation can stop an aircraft from overrunning the runway at approximately 80 miles per hour.
  - FAA Fact Sheet – Engineered Material Arresting System (EMAS)