I wouldn't go so far as to say this feature is 'hidden' within LinkedIn, but it's true to say that 'how' you can use it is unknown by many (most) LinkedIn users.
Most leads on LinkedIn are lying in plain sight, it's how you get at them which is what matters.
The key point to remember is the personalisation aspect. The overwhelming majority of connection requests I receive are not customised - not even with my name or a cheery "Hello!" so they're just not going to get my attention.
But customisation should go far deeper still, and I always recommend that before making a proactive connection request, you should take a close look at the person's profile and try to find something that you have in common - then reference it in your message.
It could be a location, a mutual connection, a previous employer or your mutual interest in Golf...
This is an important first step because it helps the recipient to start building trust in you. This, after all, is at the heart and soul of new business relationships.
Here's where you get strategic. Based on your search filters, you can drill down to where someone lives, went to school, their industry, and so on. You take that information and personalize your invitations and messages accordingly.