During the conversion of 1,3-bisphosphoglycerate to 3-phosphoglycerate and phosphoenolpyruvate, a phosphate group is added to ATP to generate ATP. The mechanism for ATP production in this reaction is

Phosphorylation

Explanation:

Here, one phosphate group is removed from 1,3-bisphosphoglycerate to phosphorylate adenosine diphosphate (ADP) to form/produce adenosine triphosphate (ATP) and 3-phosphoglycerate as product in the glycolytic pathway.

NOTE: Phosphorylation is the process of adding phosphate to an organic compound. Here (in the question), the phosphate added to ADP, makes it to become ATP.