Wiktionary
turn turtle
vb. 1 To turn upside down. Commonly used for boats and ships that capsize and turn through 180° so their hulls are above their superstructure, but it can refer to any vehicle in a similar position. 2 (context surfing English) To roll upside-down with one's surfboard (usually a longboard) to allow a wave (usually an already broken wave) to pass over.