In Situ Activation means activation of zymogens at their site of activity in the presence of their biological target by alteration in its shape.
Due to constellation of three amino acids because of unique folding of chymotrypsin, the asp 102 is able to hydrogen bond with the adjacent his 57 by borrowing a hydrogen ion. The his 57 in turn attracts a hydrogen ion from the adjacent ser 195 which allows its negatively charged oxygen anion to be able to make a nucleophilic attack on the peptide bond of the substrate.
