The bottom turn is not a manouver in surfing that judges in competitions score points on, however it probably is one of the most important manouvers that need to be executed and timed properly. The bottom turn is what set,s the sufer up to transition the rail leaning over on the inside rail , knees flexed before driving up the face of the wave to connect the surfboard with the oncoming lip to do a vertical reentry, snap or a big rail carve. In order to accomplish this the bottom turn is usually done deep almost out on the flats of the wave. You will often see surfers lean way over on their forehand with chest almost touching the water or leaning back on the heals and inside edges of the feet with their butts nearly touching. Great examples of some of the best bottom turns are surfers like Kelly Slater, Mick Fanning,Mark Occilupo, Taylor Knox or Bobby Martinez just to name a few. The steeper the wave the deeper you can bottom turn as a general rule. On softer waves sometimes it is sometimes harder to do a proprer bottom turn and the turn will be done a lot shallower. On a bottom turn you actually are temporarily slowing down your board or wiping off speed to set up for the next big manouver higher up on the face of the wave. The next time your out there and the waves are fun practice your bottom turn and see how it will help you execute other manouvers on the wave.
Stay stoked and never give up the motivation to improve!