The major reason why your hamstring cramps is because your hamstrings are being overused or over contracted due to weak or inactive glutes. Glutes are inactive because hip flexors are tight (psoas is one of the hip flexors). Here is a short blog post about what you need to focus when doing these exercises: 3 Tweaks to Help Protect Your Back from Back Pain
Just about all the movements in the routine stretch the psoas. It will take a while to feel until you can release the larger hip flexor muscle, the rectus femoris (one of the muscles of the front leg). It’s probably already being stretched, but because your other hip flexor muscles are so tight the psoas is difficult to feel.
Keep up with it and you will eventually feel that muscles stretch, but only if you follow the instructions in the link above.