how the hell does fire trap work?

I've placed it down to save my *** multiple times and they attack me right through it perfectly fine.

I've had it down and they've attacked my hero through it perfectly fine.

I've had it down or they've had it down I've attacked and it triggered on me.

I've had it down or they've had it down I've attacked their hero, it triggered on me...

Apparently I can't figure out for the life of me how this card actually works. Cause apparently it doesn't work like the card says...