A quadratic approximation improves local optimization by using both first- and second-derivative information. In XGBoost, this second-order approximation is one of the techniques used for quicker gradient-boosting optimization. section 5.4.2
Newtonian boosting update
A Newtonian gradient-boosting implementation calculates a gradient and Hessian, then trains a decision tree on the adjusted target -gradient / (hessian + reg_lambda). With a constant Hessian and regularization value 0.25, the second-order information directly influences each prediction update.
