Extended Hand-to-Big-Toe Pose (also called Standing Leg Extension or Utthita Hasta Padangusthasana in Sanskrit) is a challenging balance posture that requires the spine to remain upright with open hamstrings.
Variations of this Hand-to-Big-Toe are available for all levels. A strap is often used to modify when needed.
The standing foot is also very important, as it is the only thing making contact with the floor to support the body.
This pose is important because it creates stabilization in the body by challenging your balance on one foot. It is a strengthening posture for the legs and core in particular while stretching the hamstrings.
Since your leg is being held in the air at a 90-degree angle (or higher in some cases), it will also strengthen the ankles and the hip flexors.
It is important to remember to not sacrifice the back rounding to straighten the leg or get it higher. If this happens, bend the knee to keep the spine long.