This trigger expression works as follows:
1. Can be triggered if operations status is down.
2. {$IFCONTROL:"{#IFNAME}"}=1 - user can redefine Context macro to value - 0. That marks this interface as not important. No new trigger will be fired if this interface is down.
3. Condition of difference between last and previous value - trigger fires only if operational status was up(1) sometime before. (So, do not fire 'ethernal off' interfaces.)

WARNING: if closed manually - won't fire again on next poll, because of .diff.