What is the necessity of re-hybridisation of carbon in the transition state of SN2 reaction? This is the first time I am studying SN2 reactions and their mechanism. In March's Advanced Organic...

What is the necessity of re-hybridisation of carbon in the transition state of SN2 reaction?
This is the first time I am studying SN2 reactions and their mechanism.
In March's Advanced Organic Chemistry (The SN2 Mecahnism, page 374), it's stated:
When the transition state is reached the central carbon atom has gone from its initial sp3 hybridisation to essentially sp2 with an approximately perpendicular p orbital.
I would like to know why and how this happens. Is there an underlying mechanism? How is this rehybridisation feasible?
What makes it difficult to understand is that SN2 reactions are concerted because bond formation and breaking occur simultaneously.