On Monday, my Pilates class did this leg lift, and my abs are still sore today. I love when that happens!
It's optional to have one's head lifted off the mat while doing this leg lift/lower, so choose what suits your needs today.
To begin, start with some warm up exercises, and ensure that your spine is connected to the mat. Throughout the double leg lift/lower, the tailbone should not lift from the mat. Use that as an indicator you've reached your max if you feel the tailbone lifting.
The starting position is pictured below; sealing the legs together, lift them skyward. Contracting the abs and inhaling, slowly lower those pointed toes to any degree of difficulty, remembering to keep the tailbone connected to the mat.
Your legs may end up at a 45 degree angle, or they may be more parallel to the floor. It's up to you and your abdominal strength. Grow into the pose with practice!
When your legs have reached their target, exhale to lift them back to the starting position. Your spine should be long, your tailbone should stay connected to the mat, and your core should always be engaged.
Repeat the double leg lift/lower at least 6 times.