Computers are pretty fast nowadays. They're fast enough to spot a rocket-propelled grenade heading for your ride, and then sit back and twiddle their metaphorical thumbs for a couple hundred milliseconds before casually blowing the stuffing out of the RPG at the last possible moment, keeping you nice and safe in the process.
Unlike passive armor, Active Protection Systems are designed to proactively go out and tackle incoming threats (like rocket-propelled grenades and anti-tank missiles) before they're able to do any damage. The Israelis have already fielded their "Trophy" APS, which uses precisely targeted liquid metal jets to shred incoming projectiles.
Iron Curtain differs from Trophy in that instead of firing outwards to intercept projectiles, it fires downwards from the top of the vehicle it's protecting straight towards the ground. The system uses racks of downward firing explosives coupled with sensor arrays that track incoming projectiles, and when Iron Curtain spots something, it waits until the projectile is just inches from the side of the vehicle, and then triggers the proper explosive to explode downward, shredding the projectile into bits that bounce harmlessly off the vehicle's armor. Iron Curtain can deal with multiple incoming projectiles at the same time, and the downward blast helps to keep anyone around the vehicle from getting hit:
RPGs like the ones in the video typically travel at about 700 mph, which is no sweat for Iron Curtain. A far more dangerous threat comes from explosively formed penetrators, or EFPs, which use precisely shaped explosive charges to melt metal into an armor-piercing liquid metal jet that travels at up to 9,000 mph.
My napkin calculations suggest that at 9,000 mph, an explosively formed penetrator would cover the last three feet before impact in just under 0.3 millisecond, which is spectacularly fast and not the type of thing that Iron Curtain was designed to stop. But in a recent test, it managed to react fast enough to disrupt the metal jet anyway, which surprised even the guys who designed the system.
The Iron Curtain active protection system has been doing so well that after one last round of tests (to be concluded by the end of this year), it'll finally be heading out to Afghanistan to get to work.