Before the knock Israel calls the residents to warn them to leave.

Via CNN:

The procedure generally begins with a phone call to the occupants to leave a building, according to Relik Shafir, a retired brigadier general in the Israel Defense Forces (IDF) and a former fighter pilot.

Such places, he says, are often under constant surveillance, and the IDF has a sense of how many people live there, and how many leave.

If it is still unclear whether a building is occupied, a missile that carries little or no explosive load is aimed at the roof of a building. The impact is felt, but it rarely causes casualties.

“It’s meant to get people to take us seriously,” says Shafir.

There is no standard gap between the delivery of the “dummy” missile and fully armed missiles, says Shafir. It can be minutes or even hours. It depends on how important the target is. But there are hundreds or even thousands of such places in Gaza, chosen by Hamas precisely because they complicate targeting.

HT: The Blaze