How does captcha work for Opt-In Forms?

The captcha feature is present at all times on the Registration page (that has an Opt-In Form).

For your own custom Opt-In Forms (created in Website > Opt-In Forms), you can activate the captcha feature from Website > Settings > section 'Additional Settings' > check the box in front of the option 'Enable Google CAPTCHA for Optins' > save.

Google is using their own algorithms to determine when (in which user/subscribe conditions) the captcha test is displayed on a form.