To perform a crouch stab you need deku shield and a sword as child, or any shield and any sword as adult. Child link cannot crouch stab whilst wearing the Hylian Shield. Once those criteria are met, perform the crouch stab by shielding, untargetting, with R and pressing B. This is the fastest attack in the game to perform in succession.
For some unknown reason, the developers did not hard code the damage of the crouch stab into the tables of damage values for the weapons. Instead, crouch stabs mimic exactly the attack that preceded it. For instance, if you were to swing a hammer then crouch stab a rock with the sword, the rock would smash as though you hit it with the hammer. This is incredibly useful as child link because he can use Deku Sticks, which have an identical damage table as that of the Master Sword. So very often you'll see runners jumpslashing with a Deku Stick, and then crouch stabbing enemies with the Kokiri Sword. This does the same damage as a jumpslash with the Master Sword but costs no sticks.
This mechanic is also the main reason why ISG is such a powerful offensive glitch, because if you "store" a jumpslash and then get ISG, which requires doing a crouch stab, you can do jumpslash damage on every frame.