Squat confusion!! Tuck under or arch?

When I first started with my trainer, she told me that at the bottom of my squats, I was tucking under because of tightness in that area of my glutes. She told me that I should aim to tilt back and upwards with my hips (making more of an arch in my back). So I practiced this and she was impressed with my progress. Then one day she starts telling me that I shouldn't have so much of an arch in my back and that I should try tucking under.... I was so mad!!!! I practiced the whole tilting up movement so many times to get it right and now she's telling me something different!! Can anyone see what she might be trying to get across to me? I understand that I need to engage my core, but when I watch other people do squats, it always looks like they are tilting their hips up and they have a small arch in their back . What should I be thinking?