The video below, released by the Gaza-based news agency Watania, shows the destruction of a Palestinian building during an Israeli airstrike. The moment of impact occurs at approximately 1.20 minutes in, and leaves the building almost completely destroyed. But before that attack occurs, at approximately 12 seconds in, the building is hit by a far smaller missile.

This is the Israeli "roof knock" in action.

"Knocking the roof" is the Israeli military practice of warning the residents of a building they are targeting that they should get out. Warnings can come via a phone call or a warning missile: In this case, the occupant of the house, Samir Nofal, received both, Watania reports (it's worth noting that the video's description says the second missile strike came five minutes after the first, so the footage may have been edited). The practice has become one of the most controversial aspects of the current conflict.

When Israel and Palestinian groups fight, Palestinians are far more likely to die than Israelis are. Israel says that part of the reason for this is that Hamas and other Palestinian militant groups often store their weapons in areas where civilians live, forcing them to target these areas. In a bid to limit harm to civilians and perhaps deflect criticism, Israel has been dropping leaflets and placing phone calls, telling Palestinians to leave the northern part of the Gaza strip where the majority of their targets are. When a specific building is due to be targeted, Israel may call an occupant, or fire a small missile at the building. That's the final warning: Get out now, or you will die.

The Israel Defense Force (IDF) is open about this tactic. It recently released this video which includes a transcript of a phone call and a video of a "knock on the roof."

This video, released by the Israel Defense Forces (IDF), shows two methods for warning Palestinian civilians of an upcoming airstrike. The first part of the video features an audio recording of a call made by an IDF representative to a Palestinian citizen, warning them that they should leave the building they are in as it is about to be targeted. The second part shows the Israeli military shooting a small “warning missile” at a building in Palestine designed to warn occupants that the building will soon be destroyed. (Israel Defense Forces)

Despite the IDF's apparent confidence in the tactic, critics see flaws. The phone calls show how much of Gaza's communication networks are in Israeli control, for example, while others say that the "warnings" are not always followed up with an attack: A worrying tactic that might be considered psychological warfare.

“There is no way that firing a missile at a civilian home can constitute an effective ‘warning,’" Philip Luther, Director of the Middle East and North Africa Program at Amnesty International, says in a statement. "Amnesty International has documented cases of civilians killed or injured by such missiles in previous Israeli military operations on the Gaza Strip."

Perhaps the strongest criticism comes from human rights groups like B'Tselem that argue that, regardless of warnings, family homes are not valid military targets. Israeli officials have told the New York Times that part of their strategy is to cause "pain" to Hamas. This means targeting their Hamas leaders' homes – regardless of whether weapons are stored there or not.