It is avulsion of extensor tendon from the distal phalanx which prevents full extension of DIP leading to deformity called malley finger. It is managed conservatively in most of cases by keeping DIP in hyperextension with a brace or fixing with Kwire if bony piece of distal phalanx is large or in a high demanding patient . if PIP goes into hyperextension due to imbalanced forces it is swan neck deformity

https://studykorner.com/wp-content/uploads/2019/03/Swan-Neck-Deformity-Mallet-Finger-1200×1653.jpg

https://www.msdmanuals.com/-/media/manual/home/images/mus_fingers_abnormally_bent_bouton_swan.gif?la=en&thn=0

Extensor tendon injury opposite PIP leads to boutonniere deformity