A gedan barai is a karate

Karate

is a martial art developed in the Ryukyu Islands in what is now Okinawa, Japan. It was developed from indigenous fighting methods called and Chinese kenpō. Karate is a striking art using punching, kicking, knee and elbow strikes, and open-handed techniques such as knife-hands. Grappling, locks,...

 technique mainly used for blocking an attack. The approximate English meaning is 'low level sweep'.

Execution (Shotokan)

The following is consistent with the teachings of Shotokan

Shotokan

is a style of karate, developed from various martial arts by Gichin Funakoshi and his son Gigo Funakoshi . Gichin was born in Okinawa and is widely credited with popularizing karate through a series of public demonstrations, and by promoting the development of university karate clubs, including...

 karate. Other styles that use gedan barai may differ slightly.

From natural stance

Standing in hachiji dachi

Hachiji dachi

is a stance used in karate. In English, hachiji roughly translates to "the character for eight," but in context means something more like "shaped like number eight." Note that this refers to the shape of the kanji for the number eight: 八, not the arabic numeral "8"...

 (natural stance), and in a yoi (ready) position, bend left forearm up until fist reaches near the right ear while the right arm moves slightly in to the middle. The left elbow should be resting in the elbow pit of the right arm. Fling the left arm straight and at an angle of ~155°(vertical, up = 0°) while retracting right fist to the right hip. During this movement, your hips should swiftly rotate clockwise to add power. The rotation of the hips must be swift and stop the instant the fists stop at the final position. Another simultaneous event is the retracting hand landing on the hip, and last and most important of all, a sharp exhalation must be done through the nose with an extremely abrupt stop at the moment the fist lands. The amount of hip rotation should be about 30° (direction feet are pointing, i.e. straight forward = 0°), with the feet static, firmly gripping on the ground.

This finishes a gedan barai executed with the left arm. To do it with the right arm, simply mirror the actions.

In-depth Analysis

The gedan barai is usually the very first technique a karateka of shotokan learns. It is very basic but at the same time highly effective. The intermediate stage where the arms cross forms a very tight defence where the karateka's middle section is covered. A vulnerable spot called the solar plexus is at this moment protected. The sweeping motion also allows for a very large surface area to be covered and increases the likelihood of deflecting an incoming strike. The power generated by correct breathing and rotation of hips is very great and effective at deflecting even the most powerful of strikes. The angle at which the leading arm is likely to collide with the striking limb is such that the power is used effectively. It is likely to collide a near-right angles and so maximise deflection.

Trivia

In the most recent Mortal Kombat

Mortal Kombat (series)

Mortal Kombat, commonly abbreviated MK, is a science fantasy series of fighting games created by Ed Boon and John Tobias. The first four renditions and their updates were developed by Midway Games and initially released on arcade machines. The arcade titles were later picked up by Acclaim...

 games, the character, Johnny Cage

Johnny Cage

Johnny Cage, whose real name is John Carlton, is a video game character from the Mortal Kombat fighting game series. Cage is portrayed as a film actor who enters the game's tournament, and provides the comic relief of the franchise....

 stays naturally in a gedan barai position while using Karate. Incidentally, he also stands in a short zenkutsu dachi (front stance).