You met Adam and realized he’d benefit from a conversation with Anita. You pitch the idea to Adam and he’s grateful for the help. You know Anita really well and know Anita would be happy to speak with Adam. So you send out an email connecting them and move on.
That exchange is a “single opt in” intro. Single opt in because Adam opted in. However, Anita didn’t have the opportunity to.
A “double opt in” intro would have entailed an extra step. You would have sent Anita an email to ask her for her permission. Once she said yes, you’d then introduce Adam. This is a “double opt in” intro as you now have permission from both sides.
Double opt-in introductions are an order of magnitude better than single opt-in intros in every scenario –
(1) If Anita said yes as expected, you still gave her the choice to do so and demonstrated that you don’t take her time for granted.
(2) If Anita said no because of some unexpected circumstance, you just helped her dodge a potentially uncomfortable or embarrassing exchange.
(3) If Anita said some variant of “yes but” (e.g., yes but I’m busy over the next few weeks), it gives you the opportunity to set the right expectation with Adam.
Always choose a double opt-in intro. It is worth the extra effort.