its not for the spell its so that she can concentrate her powers Azarath Metrion Zinthos: Main incantation and mantra commonly utilized by Raven to help focus, control and enhance her tremendous yet dangerous mystical powers, usually followed by projection of visible dark energy and any of a vast number of feats; if Raven is prevented from uttering this incantation, at times she apparently is unable to activate her powers; also used by Raven and others in meditation. from the wiki
You know Beast Boy like it. After all he become a dust mite squeeze out of there easily but he chose to stay there. As for Raven why whould she sit on directly? Why couldn't she stick a cushin on him sit that. This is forplay.