[Trigger] AutoCast.

[Kwah]

How do you trigger of an auto cast ability?

 

[Just_Spectating]

unit starts the affect of ability

 

[ZiggyMcjoney]

Do you mean how you detect when an autocast's effect is fired off?

Then no, it is not starts the effect of an ability.

Theres a few ways to do it and they're all fairly dodgy unfortunately, I'm not aware of the best way. To give you a basic idea, heres a simple trigger:

Events
 Unit - A unit is issued an order with no target

Conditions
 (Issued order) Equal to "flamingarrowson"

Actions
 Trigger - Turn on FlamingArrowActions

Make the actions in FlamingArrowActions trigger off "Unit takes damage". You'll then need to do another trigger similiar to this one that turns off the trigger (the order detected would be flamingarrowsoff, in this case). Be aware that this is not MUI, although there are probably more effective ways to do this that are.

EDIT: And just as a side note, I think you can detect it with Starts the effect of an ability, but only when it is cast manually (ie, you click on the autocast ability yourself and choose the target)