A Twin Forearm Block is an example of a Diamond Block (the name more commonly seen in WTF-style taekwondo). It is a High Block performed simultaneously with a middle-height Outward Block. The fists are chambered by crossing the arms in front of the chest, palms facing the chest, with the Outward Block arm on the inside. The inside arm is brought outward while the outside arm is brought upward.
The Twin Forearm Block is seen, for example, in the form Won-Hyo.