Non-technical breakdown: The Jade uses your numeric PIN code, a secret stored on the Jade, and the blinded Blockstream server secret to spend the bitcoin.
Technical summary: The Jade has no secure element. It stores the seed information encrypted with a secret on the Jade device. But the Jade secret is also encrypted. The user logs into the Jade with a PIN code. The Green wallet software will then reach out to Blockstream servers to retrieve the decryption information for the Jade secret. The Jade secret is decrypted, and the Jade can now spend the bitcoin.