If I remember correctly from a conversation I had with a friend who works at KSC, the shuttle guidance system has some things it can do in high or low energy states on approach. In this case, if your high and fast, the HAC circle can be enlarged so the Shuttle can bleed off excessive speed and altitude. Another thing is that the landing gear can be deployed at a higher altitude, imparting drag and bringing the airspeed down. Same thing if you were "low and slow". Smaller HAC and a lower altitude for landing gear deployment.
Al