Navasana is one of those poses that look deceivingly easy to perform but in fact requires strong engagement of multiple muscles throughout your body. Most importantly the core abdominals must be strong to provide stable support to the posture.
The Navasana is approached from the seated pose of Dandasana (Staff Pose). An easier option is to bend both knees and slightly leaning back while lifting your legs off the ground. Bring your thighs closer to your chest, keeping your shin bone parallel to the ground and arms stretched forward in line with your shoulders. At this stage, there is a natural tendency to round the back and hunch the shoulders forward. Feel the grounding sensation of your sit bones and tail bone as you lengthen your spine and roll back your shoulders. Because all abdominal muscles, hip flexors, spine and quadriceps are working hard in this pose, natural flow of the breath can be restricted. Consciously ensure that you continue to breathe slowly and deeply throughout the pose.
Once you are comfortable with the first option, and your deep core muscles are stronger, take the next step by straightening both legs. This is a good workout for your quadriceps, which need to be strongly engaged to keep the knees straight.
Benefits include the toning of your intestines, abdomen, legs, thighs and buttocks. Several sets of Navasana a day keep that tummy flab at bay!