If you lean/bank on either side, you will lift one pole from the snow - instant feedback. Likewise, if you extend "up" you will lift both poles from the snow, so the major external cue here will be spraying snow from the pole tips.
Take off the pole straps and hold the poles from the handles. Press their tips into the snow, beside your boots, not in front or behind the boots.
Do a run where you focus on pressing the tips into the snow and focus on spraying snow all the way down. This will ensure the pole tips don't come off the snow and this will keep your upper body from leaning into the turn.
Note how in transition you have to stay lower (so more flexing than hopping up) and when you turn, you are more upright as opposed to leaning into the turn. Turning angles are limited because of the poles.
Progress to holding the poles from even lower, to get even more flexed. Exaggerate and hold the poles from the middle!
Here's how the pole drag will look like at a more performance level - note that the skier is holding them very low, not from the handles:
Benefits of flexing include greater edge angles and more solid carving.
Tech talks / on snow: