The first turn at the bottom of the wave after the take-off. It sets the speed and line for everything that follows.
The bottom turn is the most important turn in surfing because every move afterwards feeds off it — without one you stall on the shoulder and the wave rolls past you. Technically it runs through the rail: weight on the back foot, bend at the knees rather than folding at the hips, and build pressure through the board's edge. Once again the eyes decide it — mid-turn you're already looking up the wave face at the spot you want to hit. Started too early the turn dies halfway up; too late and you're stuck at the bottom.