INTERCHANGE OF TRAFFIC ACROSS AN INTERNET EXCHANGE POINT
A connection to an IXP does not by itself reason any traffic to be exchanged; it is a physical attendance on a common medium, not anything more.
In sort to have Internet traffic flow between two participants on an IXP, the two participants must begin BGP peering between them, and choose to proclaim routes over the peering relationship - either routes to their own addresses, or routes to addresses of other ISPs that they connect to, perhaps via other mechanisms. The additional party to the peering can then affect route filtering, where it chooses to believe those routes, and route traffic for that reason, or to ignore those routes, and use other routes to attain those addresses.
In a lot of cases, an ISP will both have a straight link to another ISP and recognize a route (normally ignored) to the other ISP during the IXP; if the direct link fails, traffic will then begin flowing over the IXP. In this way, the IXP acts as a backup link.