Back Arch
This is an exception to the flat-back rule.  Note, however, that the back is arched backward, not bowed forward.  Place your hands against the small of you back or behind your hips, whichever is more comfortable, and arch you back backwards, pushing your hips forward.  Hold for four, then release.  This is particularly useful in combination with toe touches to loosen the hips and lower back.
Various Stretches

Touch the Toes
Back Arch
Standing Groin Stretch
Hip Stretch
Hip and Hamstring
Groin Stretch
Groin Hamstring, and Hip Stretch / Straddle Split
Front Splits
Butterfly Stretch
Single Hamstring Stretch
Hurdle Stretch
Gluteus Stretch
Double Hamstring Stretch
Double Hamstring / Calf Stretch
Final Hamstring / Groin Stretch
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