This family of postures strengthens the legs, back, abdominals, and hip flexors. Variation A lengthens the hamstrings of the lifted leg, and variation B opens the adductors. Variation C asks a great deal of the psoas and rectus abdominis of the lifted leg as it strengthens the glutes of the standing leg.
While these postures do require a great deal of activation, the biggest challenge is to remain present with the breath through transitions. Remember that drishte is your friend. It is the key to steady balance. Find a point that is not moving and rest your gaze there for the duration of your stay in the posture. Utthita Hasta Padangusthasana B is perhaps the most difficult variation because the dristhe must be taken to the side at the same time that the leg is moved. Think of the leg and head as being attached to the same hinge and simultaneously swing them both open. Try to prevent the ribs from flaring out and press the hip of the lifted leg down to keep the right and left sides long.
If the lifted leg, the standing leg, or both are unable to straighten completely, do not fold forward in Variation A. Rather, remain upright in the starting position with a slight bend to the knees. You may also release the toe, bend the knee of the lifted leg, and hold your knee with your hand as an introductory modification.
Below is a video of the Utthita Hasta Padangusthasana sequence in motion. In practice, each individual asana is held for five breaths, but for the purposes of brevity, I mark each position with one breath instead of the full five. Notice as you watch that balance does not equate to stillness. Balance is fluid and responsive to the moment. Do not be rigid in your postures. Let the prana flow.