Q.Describe, in words, the geometric change at the reacting carbon during an $S_N2$ substitution -- the term for this event is 'Walden inversion'.

At the start of the reaction, the three non-reacting substituents and the leaving group are arranged around the carbon in its original geometry, with the leaving group on one side. As the nucleophile approaches directly opposite the leaving group and begins to form a bond, and the leaving group simultaneously begins to depart, the carbon passes through a transition state in which it is roughly planar, with the three non-reacting substituents spread out in that plane and the (forming and breaking) bonds to nucleophile and leaving group perpendicular to it. As the reaction completes, those three substituents continue moving in the same direction they started, ending up on the opposite side of the carbon from where they began -- much as an umbrella's canopy flips from convex to concave in a strong headwind. The overall spatial result is that the product's confi …

Describe the motion of the three non-reacting substituents specifically, not just 'the nucleophile attacks from the back' -- the invers …

Describing this only as 'the nucleophile replaces the leaving group' without conveying that the whole three-dimensional arrangement flips …