From my course notes from SQLSkills IE1, module 10 Indexing strategies (Taught by Kimberly Tripp from sqlskills.com), Slide 37, Join Strategies :

"Hash join

Two-phsae operation (build, then probe): Build table(smaller set) and probe table (larger set) allowing SQL to join extremely large sets - in MEMORY (can spill"

I just can't make this reconcile with the official answer.

Simon.