Shield
Approve the Privax Math, then call shield(amount, to). The Privax Math mints shares at the current price to to, which can be your address or a stealth address.
Unshield
Call unshield(amount, to), or sign unshieldWithSig when the shares sit on a stealth address with no ETH. Anyone can submit the signature; the submitter earns the fee you signed.
Shields and unshields are public
The address that shields, the address that receives an unshield, and both amounts are visible. Privacy comes from what happens in between.