Drop-in anchors are deformation-controlled expansion anchors designed for attaching various decorations, utility or construction elements to a solid base material such as concrete or masonry. This anchor is composed of a hollow sleeve and an internal expander plug as shown in the following picture:

A hollow sleeve

Four equally spaced slots cut longitudinally to create an expandable shirt at one end.

With internal threads at the other end of the sleeve to accept a bolt or a threaded rod easily.

Special patterns for higher holding strength as it increases more friction between the anchor and drilled holes.

An internal expander plug 

Located inside the sleeve above the slots, this solid, cone-shaped plug is gone through case hardening and tempering to avoid galling and binding during expansion effectively. It works by forcing the expandable shirt outward for locking the anchor in place.