Sometimes a role must wait for something to happen before it can proceed. To be more precise, sometimes a thread must wait for something to happen before it can proceed. We call that something a trigger.
We show a trigger by an arrow () placed on the state line — see Figure 2.1— with a caption that briefly describes the trigger concerned. In strict terms, a trigger moves the role concerned from the state preceding the little arrow to the state just after it — imagine this in terms of the movement of a token. In Figure 2.46 we see the start of a thread that waits