Often this arm balance is referred to as bakasana (crow), whether the arms are bent or straight; however, there is a difference. In kakasana (crow) the elbows are bent, and in bakasana (crane) the elbows are straight.
Setup and Key Actions
From tadasana (mountain pose) with feet hip-width (about two-fists’ distance) apart, bend your knees, plant your hands on the floor, and lift up onto the balls of your feet. Spread your fingers evenly apart, resist your thumbs toward your fingers, and your fingers toward your thumbs (like you’re clawing the floor), and press your index knuckles firmly into the ground. Bend your elbows (back toward your body, not out), and bring your knees up high on the backs of your upper arms. Start to shift your weight forward. Lift one foot at a time, with toes spread. When the second foot lifts, squeeze the inner edges of both feet together. Push the floor away; round your back, and move your chest forward.
Once you've got the hang of coming into crow from tadasana, you can explore coming into it from a low squat (malasana), a variation that provides and requires a much deeper hip stretch. Start in the deep squat. See if you can keep your inner feet squeezing together, heels lifted off of the floor, knees apart. (Starting with your inner feet squeezing together will help you to "hug to the midline" from the get-go, making the pose more stable). Walk your hands forward to create length in your body, then walk them back toward you, bending your elbows and walking your knees up high onto your upper arms. From here, stay low, and start to shift your weight more onto the balls of your feet. You may even be able to lift both feet together. If not, lift one at a time, and squeeze the inner edges of your feet together once you get up. Push the floor away; round your back, and move your chest forward.
To move from crow into crane, keep your hips lifted, your back rounding, and your chest reaching forward as you draw your heels up toward your seat and continue to push the floor away as you work to straighten your arms.
You can also try coming into crane with straight arms from the get-go, perching your knees high up onto your straight arms and shifting your weight forward to float your feet off of the floor.
If liftoff is challenging, try starting with the ball of one foot up on a yoga block. (As you play with balance here, experiment with lifting the foot on the block first or lifting the foot on the floor first and see which works best for you). If you're hesitant to shift forward for fear of falling on your face, practice with a bolster or stack of blankets in front of you.