K-Files with small diameters have a rectangular cross--section to improve torsional strength and fracture resistance.
However, their felexibility is quite limited.
Flexile files have a triangular cross-section
This decreases cross-sectional 2-D moment to provide better flexibility than files with a rectangular cross-section
Mani Flexile Files
Why to choose Flexile Files instead of K-Files?
The flexibility of triangular cross-section(Flexile File) type is about twice that of Square cross-section (K-File)
Increased cutting efficiency as compared to K-Files