Japanese: M字開脚縛り
M-Ji (M字): M-Shape
Kaikyaku (開脚): Spread legs
Shibari (縛り):  Tie

A classic seated ankle to tied to thigh making the model resemble the letter M with the open legs.

Also known as: “M-ji shibari (M-字縛り)” 

See also: M-ji-kaikyaku zuri shibari