Create a typical form submission trigger email. Go to the 'Settings' tab. Go to 'Filter Recipients'. This allows you to filter the type of person who will receive this triggered email.
Select the field you want, select the function you want (equals, etc.) and fill in the value.
Example 1: Rule 1 - OptIn - Equals - Yes. Anyone who inputted that field with that value will get the email.
Example 2: Rule 1 - OptIn - Not Blank. This makes the trigger email occur if someone fills out that field, no matter the value selected.