Why should you do Stiffed Leg Deadlift ?
WHY YOU SHOULD DO STIFFED LEG #DEADLIFT (SLD)
It is important to incorporate different types of hamstring exercises in your program.
NSCA (1999) found that Single-joint hamstring movements like the SDL and the leg curl have been shown to produce approximately twice as much hamstring muscle activation as the squat. 
Hamstring exercises that involve breaking a the hips, like the Romanian deadlift and stiff-leg deadlift, cause significantly greater muscle activation in the upper hamstrings than the lower, while hamstring exercises that break at the knees, like the lying leg curl (movement originating at knees) cause significantly greater muscle activation in the lower hamstrings. [2, 3]
This is why it's important to incorporate both types of hamstring exercises in your program. An exercise that breaks at the hip and one that breaks at the knees.
Training hamstrings is also important because strength imbalances between the hamstrings and quadriceps may increase the possibility of hamstring injuries. 
When performing any deadlift exercises, avoid *hyperextending* as this can lead to lower back injuries. You can always record a video of your deadlift from the side to see where you are finishing.
PERFORMING STIFF LEG DEADLIFT
Breath in your Core
Retract your shoulder blade
2. Coaching point
Brace the core and Squeeze the glutes when go down
Go as far as you can without rounding your back and feel the stretch on your harmstring
Drive your hips forward by squeezing your glutes . Pull through your hamrstring
Don’t hyperextend your lower back when you come up
3. https://www.researchgate.net/publication/263548541_Regional_Differences_in_Muscle_Activation_During_Hamstrings_Exercise (full-text of study)