How would you describe moving your head back in shock? [duplicate]

I would say "flinch back" or, as mentioned by FF, "recoil".

• "she flinched her head back in horror"
• "she recoiled in fear when she saw his face"

• If you flinch, you make a small sudden movement, especially when something surprises you or hurts you.
  Murat had looked into the eyes of the firing squad without flinching. [VERB] The sharp surface of the rock caught at her skin, making her flinch. [VERB] (Collins)

• If something makes you recoil, you move your body quickly away from it because it frightens, offends, or hurts you.
  For a moment I thought he was going to kiss me. I recoiled in horror. [VERB]
  We are attracted by nice smells and recoil from nasty ones. [VERB + from] (Collins)

Examples from the web:

• He flinched his head back inside the cab.

• Victoria flinched her head back as if Tracy had severely insulted her or slapped her on the face. "How dare you attempt to tell me how to raise my son!"

to shrink is a good word: TFD]1 And the idiom shrink back TFD

to draw back; recoil: to shrink from danger.

But, it does not imply just ones head, but rather the whole person.

To use the idiom:

Viewing the crowd, heads (most visible part of the many bodies) were seen shrinking back from the new danger.

In literary use, my sense is this would be acceptable. I do not know of any specific phrases, including those discussed in this question, are specific to the head, especially its motion or reaction to 'shock'.