Well it turns out that I misunderstood and wasn't supposed to keep my feet straight. Apparently my toes were flaring out too much and the angle was too wide, with my heels too close to my body, and I need to make sure I can put my butt between my heels to maximize glue ham and quads. My mistake.
I'm amazed at how many nuances there are with Olympic lifting, and how helpful it is to have someone with a trained eye point things out.