How do I make the private key on a OpenPGP smartcard non exportable ?

[T L]

Under GPA in windows there is a option to backup the private key from a
OpenPGP smartcard. My understanding is that one of the main purposes of
smartcard use is to prevent the private key from being exported and force
cryptographic operations through the card. Is there a method of setting the
private key non exportable that I am unaware of ? If not then what am I
missing ? How is this more secure than a password protected file ?
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