With presentational acting, the actor acknowledges the audience. With representational acting, the audience is studiously ignored and treated as voyeurs. Conversely, the type of theatre that uses 'representational acting' in the first sense is often associated with a performer using 'presentational acting' methodology.
Presentational, it's kind of in the name. "Present" is how I like to think of it. It's when the actor acknowledges the audience, realizes they're there. Representational is almost like the audience members are looking in on a character's life, the actors "don't know" the audience exists.
