Placing of the hands on the mat!
Placing our hands; without correct alignment, our shoulders and heart will not have access to deeper movement. Here you can see the difference between alignment that will create impingement in the wrist.
There are four points on the palm of the hand that lay equally in the earth. The wrist is parallel with the front of the mat, and this will be the same for all poses in which the hands are in this position (for example, downward facing dog pose, handstand, chataranga dandasana and of course bhujangasana). We want to make sure that there is no collapse of the retinaculum (the centre of the wrist), which should instead be like a keyhole, and that there is energy being drawn up through the arms, out of the earth, into the bottom of the heart. The back of the body is filled even as the heart surrenders and opens.