Take a knee. One foot forward and the other knee on the ground. Keep your hands to the sides, don’t put them on your legs! Keep your torso straight up. From this position lean towards your knee without leaning your torso forward.
You can also experiment with rounding your back a bit when doing this (most people have a tendency to arch it) in order to better feel the hip flexors.
In order to use the tense-relaxation technique described earlier in this book do the following: start as explained above and lean into the stretch. From there statically ‘kick’ with the knee on the floor. The muscle will tighten up. Hold that tension for 5-10 seconds. Hold your breath while doing it. Then suddenly relax and breathe out simultaneously while sinking deeper into the stretch.