"the tutorial in text"
1. i will show you how to make a headcrab hanging from ceiling
2. create a npc_headcrab_fast on the ceiling.
3. now go into "flags" and uncheck "keep corpse" and check "start hanging from ceiling"
4. find the trigger-tool and make a box were you want the headcrab to start dropping down
5. press ctrl-T and make it to an "trigger_once"
6. give your "npc_headcrab_fast" a name
7. go to your trigger and go to "OutPuts"
8. press "add..." - "OnTrigger" - "(NameOfTheHeadcrab)" - "DropHangingFromCeiling" - press "apply"
9. now your done!