Hey nowon, can you tell me what this event is in your cap file?
It's called an inverted condition. So it will be true as long as Block does NOT overlap Terrain. You can rightclick a condition and choose "Invert condition" to invert/negate it.
Also let me mention that the always condition in that first event is unnecessary . You can as well leave it out. The event will run every tick nonetheless.
More importantly in Nowon's example and your cap you're just adding to the Y position without incorporating TimeDelta (it would have to be like set Y position to object.Y + 'Y Speed' * TimeDelta). Which would mean the blocks will fall at different speeds on different computers depending on the refresh rate.
So this is really a thing you should keep an eye on. I recommend reading the following wiki article: TimeDelta Wiki