Starting Strength: BBT, 3d ed. (p. 86-87):
First, "squeeze your quads tight to lock the knees hard in extension, and then squeeze your abs tight to set your lumbar spine from the front. This squeezing from the ribcage down to knees creates a band of tension that we will use to create a rebound. Once this tension is build, push your hips forward so that your weight goes onto your toes. This movement must not be produced by bending the knees . . ."
This recent video from Coach Socolinsky and Coach Socolinsky recommends a bit wider stance with toes out, in order to guard against bending the knees:
Common Press Mistakes and How to Fix Them | Diego and Emily Socolinsky
With heavier weights, any distinction between locking your knees and not bending them probably becomes less noticeable, as you work harder and harder to stay tight under the bar.