Most people think a lunge is an upright movement with the torso perpendicular to the ground throughout. But that's flawed mechanics. Maintaining a forward torso lean onto the heel of the front leg is essential.
The forward lean ensures the hips are pushed back posteriorly and actively engaged throughout the exercise. A good lunge involves solid hip hinge mechanics. An upright torso places greater stress on the knees and low back while minimizing stress to the glutes and upper thighs – not what you want.
When doing a stationary lunge, the torso should move straight up and down while keeping the forward lean. Any horizontal displacement of the torso indicates faulty hip mechanics and lack of spinal rigidity.