Brudzinski’s sign

Even though Brudzinski  (a very famous Polish Physician)  described several signs in patients with meningitis, Brudzinski’s neck sign is most the most used test and is  referred to as Brudzinski’s sign.
How to elicit this sign?
With the patient supine, the examiner places one hand behind the patient’s head and places the other hand on the patient’s chest. The examiner then raises the patient’s head (with the hand behind the head)
while the hand on the chest restrains the patient and prevents the patient from rising. Flexion of the
patient’s lower extremities (hips and knees) constitutes a positive sign. Brudzinski’s neck sign is more sensitive than Kernig’s sign. 
How is it caused?

This sign is caused by the  motor nerve roots irritation which pass through inflamed meninges because the roots are brought under tension.

Image from Wikipedia: Meninges of the central nervous system: dura mater, arachnoid, and pia mater.