In the event the "hello" packets are not received on an interface, or an interface waiting for "hello" more than 2.5 minutes after the other interface went into normal state, the interface is placed in "testing" mode (if the interface is not shutdown and link status is up). When this occurs, the other unit is informed through the failover cable that the interface is in testing mode. While an interface is in testing mode, normal traffic can flow, provided the interface is functioning properly. Testing is started only if an error condition has occurred and is therefore based on the idea that "if I'm okay, then you must be failed."
Hope that answers your query.
R/Yusuf