Bassai means "to penetrate a fortress". This kata gets its name from its opening move, where the practitioner uses all his/her power to explode forward in a powerful move, as if breaking down a door. According to Funakoshi-sensei, the constant switching of arm positions is meant to teach the student the feeling of shifting from a weaker to a stronger position.
The line of movement used is the "T".
For sample moves from the kata, click here.