Corner Seated L Sit Single Leg Raise. How-to, proper form and tips

Sit down on the floor, your back on the wall and raise one leg in a straight position.
Video demonstration
(excerpt from 'Seated Straight Leg Up'
by FitBest)
How to and step by step instructions:
- Sit down on the floor, your back on the wall.
- Make sure your back and shoulders touch the wall, as well as your butt.
- Straight your arms placing hands on the floor, by side with your quads.
- Raise one leg keeping it in a straight position.
- Count the number of seconds you can hold this position.
Hints & tips:
- Hamstring flexibility is needed for the exercise.
- Start with a few seconds and progress from there.
Benefits:
- Improves posture of your back.
- Highly activates core, hamstrings and quads.
- Starting point for the L Sit.
Counting & gamification:
- Try similar times with each leg, give precedence to your weaker leg.
- Count the seconds you can hold, accumulating in a period (session, day, week).
- Set an accumulated goal for a workout session, say 60 or 120 seconds, and do the necessary repetitions to reach that goal.
Muscles worked and body parts involved:

Primary muscles: Lowerback, Hamstring
Secondary muscles: Quads
Seated Leg Raises, easy to hard

2. Corner Seated L Sit Single Leg Raise