The figure 8 on a bight is a very popular rescue knot. It is easy to tie, easy to inspect, relatively easy to untie after being loaded, and it is a member of the versatile figure 8 family of knots which makes it easy to remember.
To tie a figure 8 on a bight, take a bight of rope and tie a figure 8 knot using the bight. This is very similar to a figure 8 skeleton knot, except you tie the knot with a bight. Dress the strands and leave a tail that is at least fist-width (3-4") long.
The "critical point test" (aka "scissors test") requires that if any one item fails, nobody should be injured.
A "bend" is a knot that joins two ropes.
A "hitch" is a type of knot that must be tied around another object.
The "whistle test" requires that if all rescuers let go, nobody should be injured.