It is not like in Canada, where simply showing your Nexus card gets you into the expedited security lane.

You must follow the instructions to add your KTN (i.e. PassID from your card) into your reservation. When you check-in, your secure flight info. gets sent to the TSA, and they either allow pre-check or they don't. If they allow it, the info. gets transmitted to the airline, who embeds this in the barcode on your BP, and so when it gets scanned at the checkpoint, they know you have it (or don't have it), and can direct you to the appropriate place.

Note the word they use is "eligible" - you do not necessarily get it everytime. As a member of Nexus, I rarely don't get pre-check. But it happens, most likely on a random basis, because I always get it back the next time.