This is what I understand it to mean based on a brief internet research: The presenting head has entered the true pelvis.
Some natives may hopefully weigh in and confirm or tell us otherwise.

Esame ostetrico
4° manovra: per accertare l'impegno della parte presentata, applicando le mani come nella 3° manovra, s'imprimono alla parte presentata dei movimenti di lateralita, se la parte presentata e fuori scavo è spostabile, viceversa se è fissa vuol dire che è impegnata.
https://issuu.com/antoniobifaro/docs/tascabile_ostetricia/35

Step 3. Using the thumb and fingers of one hand the lower abdomen is grasped just above the pubic symphysis to establish if the presenting part is engaged. If not engaged a movable body part will be felt. The presenting part is the part of the fetus that is felt to be in closest proximity to the birth canal.
http://www.perinatology.com/Reference/glossary/L/Leopolds.ht...

Engagement [en-gāj´ment]: the entrance of the largest diameter of the fetal head into the smallest diameter of the maternal pelvis
https://medical-dictionary.thefreedictionary.com/engaged