The manual states....
Walkaway time begins when
an agent puts a call on Hold, and hangs up or unplugs the headset
an agent in Not Ready state puts a call on hold
Walkaway time ends when the agent takes the phoneset off hook or plugs in the
headset.
Generally speaking, it's when an agent has a call and needs to get something to answer a question that they haven't got on their desk, so the put the call on hold, break the headset cord connection, get the paperwork, or whatever, come back, re-connect the headset cord and take the call off hold.