Welcome in the dictionary of the terms Long Board surfing
Turtle
The duck is impossible for the majority of the longboarder, only is practised the turtle.
According to councils' of other practise, under water and thus under the board, to tighten the arms by clutching the board (which is with back) while letting derive my body, that shook a little but it is rather effective, one moves back but not too.
The turtle, I have perso tested at the beginning... but this manoeuvre is rather tiring, is necessary to go up on the board after each vagueness, to go down again from there of continuation afterwards for the following one especially to the Mediterranean with short series waves.
My technique of beginner, I try to cross foam while pulling up and while making the rocking horse, by amplitude of 1 with 1m5, that passes (often).