Brakesliding is a precise form of movement that can be performed after a roll. An optimal brakeslide starts at -23 speed, slowly decreasing to 0. For comparison, a roll is 26 speed.
Since a brakeslide is slower than a roll, it should only be performed in instances where you still have some distance to cross, but not enough place to do a roll. A good exemple of a place where it can save time are inside dungeons, when you want to reach a door but are in a position that would cause you to bonk if you do a roll. Doing a brakeslide instead of walking usually saves 2 or 3 frames in those circomstances.
Due to how precise the timing for an optimal brakeslide is and how little time it saves, it is usually not used in real time runs.
This trick abuses something called the ESS position. ESS stands for Extended SuperSlide Position, it is smallest input of the control stick that the game registers as not being neutral. In particular, this trick uses ESS Down, which is the closest your joystick can be to neutral while being below neutral, and the game considering you not neutral.
To perform an Optimal Brakeslide, perform a roll and press L+Down on the first frame after the roll animation ends. Then on the very next frame hold L+ESS Down. You will be sliding at -23 speed, slowly decreasing.
To perform an Extended Brakeslide, start just like an Optimal Brakeslide. Do a roll and press L+Down on the first frame after the roll animation ends. But this time, on the very next frame only hold ESS Down. The only difference in inputs between this and an Optimal Brakeslide is that you release L on the 2nd frame. An Extended Brakeslide makes Link travel backwards due to the fact you are travelling with negative speed without holding target. You also lose speed much slower due to not holding target.