Slingshot pitching motion Legal or illegal?

[finfan365]

This is my dd first year of MS softball, so we are new to NFHS rules. She will occasionally throw what I believe is referred to as a slingshot pitch. Her motion is essentially a C shape where she winds up and delivers without a full forward rotation. On two occasions this season umpires have questioned this. One umpire declared it as an illegal pitch. A second, last night, warned her coach that she must complete 3/4 of a circle motion before delivery. In nothing I find online, have I seen mention of a required minimum rotation. I have seen rules against a second rotation after release, but nothing states that a pitcher must complete at least 1 circle.

Does anyone have clarification on legal vs illegal pitching motion?

 

[BretMan2]

Perfectly 100% legal.

 

[DOKelm3]

It is legal, had a girl who threw a changeup that way against us. It was a very popular pitch in the past, it is died out

 

[USAUmp1]

Slingshot is a legal pitch as long as she doesn't do something to
make it illegal.