Katana Mist Slash

 A move by which, like a sickle mowing grass, the practitioner advances one step with the forward leg and delivers a slashing blow to the opponent's side with the palm of the hand.

The movement and stance, indeed the name itself, suggest that this move evolved from a sword drawing move to one used in barehanded combat.

In swordsmanship, horizontal cuts were ineffective because they were blunted by body armor, so it is likely this move was developed relatively recently by the Hazuki Style. If an opponent is focused, he can withstand a blow delivered to the front of the body, but the body is surprisingly vulnerable to glancing blows.

In the Chinese martial arts, blows delivered with the palm, called Cutting Palm Strikes, were one type of the so-called Ultimate Power blows, and many examples exist in the oral tradition. Delving deeply in the mysteries of the ancient martial arts reveals the existence of moves involving the use of secret weapons called the Dark Weapon and the Unrevealed Weapon.

Originally, the Katana Mist Slash was likely one of these moves involving a secret weapon to enhance its effectiveness.

Move Origins

Though not mentioned outright, this move likely originates from Aikido. The move description mentions the technique being added to the Hazuki style relatively recently and fits with the development of Aikido which occurred in the early 20th century. Aikido is also famous for their strikes resembling sword or weapon cuts (possibly as an adaptation or holdover from earlier jiujitsu when the techniques were used in armed combat). It could also be a case of convergent evolution canon-wise as the Hazuki Style has a lot of similarities in terms of development as aikido.

Usage of Open Hand Chops

Striking to the side of an opponent's body can be very effective. The ribs aren't covered by much muscle and can be broken with a strike, severely impacting breathing. A good strike to the liver can shutdown one's body, effectively knocking someone out. But why use an openhanded chop at all? These openhanded chops (knifehand strikes or shutō-uchi in Japanese) have less risk of damaging the hands, making them more useful in a bareknuckle context. Boxers can target the ribs with impunity wearing gloves, but a bad connection might hurt their hands instead if they weren't wearing any. Even with gloves, the risk isn't zero as Arturo Gatti found out when he broke his hand punching Micky Ward's hipbone.

Secret Weapons

The most intriguing aspect is the mention of "secret" weapons in the move description. Secret weapons could refer to multiple things historically in Japanese martial arts:

1. Small, concealable weapons meant for backup/last ditch use. 
2. Weapons disguised as ordinary objects (such as pipes or fans) that could be carried in environments where weapons were not allowed
3. A weapon that a ryūha (Japanese martial arts style/school) secretly specialized in. Ex. a school known for their sword use secretly training in the use of another weapon like the spear.

The secret weapon in this case most likely refers to a weapon of the first type. One type of secret weapon was the shutōgane, a small bar of iron worn on the hand that enhanced the effect of shutō strikes. A chop to the ribs barehanded might break a rib, but a chop with a shutōgane makes the possibility much more certain.

A shutōgane from the collection of Serge Mol, Classical Weaponry of Japan