From lots of sitting or shoes with a heel (did I mention most shoes have a heel even ‘flats’ and trainers?) or just from poor postural habits you can develop a constant slight knee flexion. The issue with this is that gradually this constant flexion shortens the calf and the muscles in the back of the thigh and over time will create a pull on your pelvis into a posterior tilt. And that posterior tilt will among other things compress your lumbar vertebrae causing back pain and prevent your pelvis from being as mobile as it needs to be which is specially important if you are about to give birth.
The double calf stretch is what you need to create length in the muscles down the back of your legs. something to think about when looking at untucking your pelvis in the exercise, if you are finding that a hard concept, is instead to think about allowing the bony front parts of your pelvis (the anterior superior iliac spine or ASIS) to relax towards the floor – don’t force it though just breath and allow it to happen.
Once you start creating length in your calves and hamstrings you can then start to strengthen them and your gluteals. Strength here is what is needed to hold your body upright and propel you forward. An important thing to remember is you can’t get strength without length.