He starts it off with a great flexibility drill using Wall Bars (every gym, no matter weightlifting/gymnastics/crossfit should have some, seriously). This one is a staple in gymnastics training.
It’s basically a weighted Pike Stretch. You increase the load by grabbing lower and lower rungs.
Of course this is not only useful for gymnasts. It will also help a ton if your crappy hip flexion is giving you a hard time during squats.
For a novice, start with feet on a low rung, legs straddled at about 90 degrees, and hands grabbing up high. Keep those knees and arms locked out and push your butt back to allow for the stretch. You will feel the stretch not only in your hamstrings but also all along your back and shoulders.
As you get more flexible, grab a lower rung to work on a more compressed position.
The second stretch (inverted version) is a little more advanced, and will take a little longer to get in position. I would recommend putting a soft mat underneath the stall bars just to be safe.