Undercut anchors are not your typical anchors. The bottom of the hole has a notched opening, or undercut, made either with a special drill bit or with an anchor that has a drilling device at the end of the bolt. When in place, the anchor uses the compressive strength of the concrete to resist the load, unlike a wedge anchor, which relies on the concrete's tensile strength. Undercut anchors are the strongest you can buy, and you put them in places where you want to avoid problems no matter what the cost. That is why they are commonly used on roller coasters, nuclear power plants, and other structures where human safety is paramount. They are well suited for dynamic loads and are frequently employed in seismic retrofitting. The anchorage is so strong that the steel in the anchor will often break before the concrete.
There are two types of undercut anchors. The standard type requires drilling the hole, followed by drilling the undercut with the special bit. The second type is called a self-drilling anchor, which doesn't require the additional bit.