This is what the top looked like in the morning of day 3 before it was fed. When you stir a starter that has risen up like this, it'll deflate and have a light, airy "honeycomb" texture. (But just remember, this isn't a strong starter yet, it still has the smelly bacteria making it look like this.)
What you're aiming for, eventually, it a predictable rise and fall every 8-12 hours. You want to feed it "at the top" or just when you see streaks that show it's starting to fall. That's when you know you're on the right track.
Image Source: POPSUGAR Photography / Tara Block