You set EFPs off with the detonator, either via wire, or wireless, connection. When the C4 explodes, it forms the copper cap into a blob of molten copper, moving faster than a speeding bullet (about 1,500 meters a second). The blob stays intact, and lethal, for a few hundred meters, traveling pretty much in a straight line. However, the EFP is still difficult to aim. The user has to place it so that, when it goes off, it will hit a vehicle sitting in a position the user has already figured out. For this reason, EFPs are usually set up at places where vehicles have to stop.
When the EFP hits an armored vehicle, it burns and punches its way through the armor. Once inside the vehicle, it injures or kills whoever it hits, as well as igniting combustible material and generally scaring the hell out of everyone. The increased use of MRAP vehicles however, has meant that, while EFP attacks are up over a third this year, casualties from those weapons is down 17 percent.
Good story. Well worth the visit to read the entirety.
As is usual in war, both sides are trying to win some advantage. The (shrinking number of) insurgents are trying to use EFPs in innovative ways to maximize their impact. The coalition forces take note of what the insurgents attempt, underline what works, and attempt modifications in armor or tactics to neutralize the enemy strategy.
MRAPs have proved an important technological advance that has significantly assisted military operations in Iraq--not a "Tool of war trumped."