This exercise may be done with or without an exercise band to increase resistance. It's completely up to you.
The Frog works the core as well as the legs. It encourages you to utilize the major muscle groups in the legs (calves and quads) and to engage the core for stability.
To start, find your neutral spine; your shoulders and tailbone should be connected to the mat. Each vertebrae should be nestled into your mat as you prep. Legs lift with knees bent, whether or not you're placing the band across the balls of your feet. The heels should be touching and the toes should be turned out, NOT touching. Knees should be about shoulder-width apart when they are over the torso.
If you're using the band, keep your elbows planted at your sides while pressing the legs long. These photos depict the model lifting her elbows off the mat; I find this may hinder your ability to maintain proper posture. While extending those legs, be sure your tailbone does NOT come off the mat. You can choose whether or not the head is lifted in this exercise.
Your legs can extend at any degree of difficulty that you'd like depending on the challenge you need. As you extend, the knees come together while you maintain turnout in your feet. Use your exhale to extend and your inhale to pull the legs back into the chest.
Repeat 6-8 times, building up your stamina and challenging yourself to eventually extend the legs closer to the floor in the future.