Twin Hand Waves

Abridged move description courtesy of Suka Pass:

This move involves advancing a step forward and striking the opponent's chest and jaw with the palm and entire forearm while deflecting the opponent's attack downward with the other hand. This doesn't merely involve hurling the opponent's body backward as with a sumo arm thrust. The impact creates a shockwave that affects the opponent's internal organs.

Originally, it was a move devised in the Warring States Era to strike down samurai wearing armor. Not only was it ineffective to strike the hard torso of an opponent's armor with the hardened edge of the fist or elbow, it also injured the practitioner. For that reason, the softer, fleshier palm of the hand and the entire forearm was used to attack.

I was unable to find an exact 1:1 copy of the move, though there are a fair amount of similar looking moves in traditional martial arts (comment below if you know any real life moves that resemble the in-game Twin Hand Waves).

Top: Xing Yi Quan style splitting technique

Bottom: A forearm strike done during a Tai Chi push hands drill

The idea that the palm and forearm were used to avoid injury to the practitioner does hold some merit. While the description mentions the move being used against a person in armor, hitting even an unarmored person can cause injury to the user's hands if struck wrong. Many boxers in the bare knuckle boxing bouts the 18th and 19th centuries were common working men and could not take matches often as their hands would be too injured for them to use at work the next day. In modern revivals of bare knuckle boxing such as BKFC, broken and/or extremely swollen hands are common injuries. The palm and forearm are comparatively less fragile.

Palm strikes are effective strikes and Ryo has quite a few in his repertoire, so we'll be saving those for a future installment and instead focus on the forearm component of Twin Hand Waves. 

Forearm strikes are rare in modern striking competitions. In boxing, intentional forearm strikes are illegal and forearms are mainly used for defensive purposes to help shield against incoming punches. Though striking with the forearm is illegal, they can still be used to create opportunities to continue punching.

Though it doesn't count as a strike if you hold your forearm out and they run into it, as Conor McGregor discovered.

Floyd Mayweather Jr is a master at using his forearms to control his opponents. He creates a frame with his forearms that he can use to place on and push against the opponent's head. This framing gives Floyd control of the opponent's head and makes it very difficult for them to move effectively (as in the old grappling adage "control the head, control the body"). Floyd can use this to deny an opponent from countering him after he has thrown his combos, prevent or be used to break free from a clinch, and can smother an opponent's hands for a free shot at their unguarded head.

In this sequence, Conor attempts to enter a clinch after avoiding a flurry from Floyd. Floyd uses his left forearm to frame on Conor's head, preventing him from coming forward. Simultaneously, Floyd's right hand covers Conor's left, preventing Conor from throwing his vaunted left hand or link his hands together for the clinch. With Conor's left hand covered, Floyd throws a right hand at Conor's now-unguarded head without fear of reprisal.

Even in striking arts where forearm strikes are legal to use, they are still pretty uncommon.The use of forearms in sports such as Muay Thai or MMA trends more towards defensive uses such as blocking or framing as it does in boxing. At the range that forearm strikes are used, elbows tend to be more effective due to being able to generate more power or potential cuts (as well as being legal in nearly all the competitions that forearm strikes would also be legal in). The forearm strikes I've seen in MMA and Muay Thai seem to occur off of missed elbows or punches rather than intentional use, though committed attempts at forearm strikes do happen on occasion.

Former Rizin/Bellator BW champion Kyoji Horiguchi uses a lead forearm strike to stun Ali Bagautinov

Despite the description for the move explicitly mentioning it being different from sumo arm thrusts (called "tsuki-dashi" in sumo), the person closest to using Twin Hand Waves in real life is ironically a sumo wrestler. Hakuhō is arguably the greatest sumo wrestler of all time, having reached Yokozuna the highest rank possible as well as holding multiple all-time wins and championship records. One of Hakuhō's effective opening move combinations is the harite (open hand strike or slap) and kachiage (forearm strike).

Hakukō parries the initial charge and drives a forearm into his opponent's chest, forcing their posture upward and leaving them more susceptible to follow-up attacks

While technically legal, it's frowned up in sumo, especially by a wrestler of Hakuhō's caliber. However, it's practically as close to the in-game description as you can get and damn does it work.

Another example of Hakuhō slapping the head of his opponent to deflect the momentum of their charge and driving in his forearm. This time, his forearm connects with his opponent's chin, leading to a knockdown.