Q: My students have beautiful alignment when standing still, but when they start moving, their abdominals become like jello. What gives?
A: When a student stands tall, lifting their ribs and lengthening their spines (as if they've put an invisible belt around their waists), it looks like they are in proper alignment. But actually, they're often holding their breath to maintain this position, and when they finally release their abdominals to allow air to come into the lungs, jello-like movement can occur. I suspect this is what is happening with your students.
So how can we train the abdominals to stay still? First, teach your dancers to get familiar with how the abdominals stabilize the pelvis. Have your students rest on their backs with their legs bent and the soles of their feet on the floor. Ask them to knit their abdominals together while sliding one leg out straight in front of them. Repeat on the other side. Your students will feel the engagement of their abdominals primarily below the belly button. They should still be able to breathe fully, and their abdominal muscles should naturally contract during the exhale.
Once you've done this, ask your dancers to stand up and draw the front of the pelvis upward toward the breast bone without lifting or dropping the ribs. Again, they will feel effort below the belly button. Have them walk around for two minutes while keeping the pelvis level and the spine elongated—it's harder than you might expect. This sensation of engaging the abdominals is what your dancers should aim for when moving. Strengthening these muscles will keep them from looking like jello in class.