Jerusalem (CNN) -- It's become another military euphemism, along with "collateral damage" and "friendly fire."
The "knock on the roof" is the Israeli military's warning for civilians before it fires on a building and is being used extensively as Israeli airstrikes target Hamas sites in Operation Protective Edge.
The Israeli Air Force developed the technique in 2009 as a way to warn civilians in Gaza to leave buildings it has identified as locations where Hamas keeps ammunition, a rocket stash or command post. But it is a controversial policy that has been criticized by human rights groups.
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.
One dramatic example of the "knock on the roof" appeared on social media Sunday. Distributed by the Gaza-based Watania news agency, it showed at first the impact of an unarmed rocket or missile on the roof of a house. Smoke rose from the roof; curtains billowed from an open window.
Some time later -- Watania says about 15 minutes -- two missiles slammed into the building, blowing out the facade and sending debris and thick smoke across a wide area.
It's not clear whether the occupants of the house had heeded the warning -- it may have been empty -- but no one was hurt.