Bredt's Rule | bridged systems

Bredt’s Rule states that bridged systems having a double bond at the bridgehead position cannot exist.
Thus according to this rule introduction of a double bond at the bridgehead of a fused ring system by any means is impossible.
A simple example, which illustrates Bredt’s Rule, is the dehydration of Norborneol (Bicyclo- [2,2,1] heptan-2-ol). This reaction is an elimination reaction wherein the substrate (norborneol) possesses two beta-carbons (the bridgehead methane carbon (C1) and methylene carbon (C3)), both of which bear hydrogen which can be competitively abstracted and thus the double bond may be oriented in either of the two ways to yield two products.
Thus the product as per Saytzeff’s Rule should be the major product. But this is not observed, as the product is not formed, as the elimination is not from the bridgehead carbon. The other product (alkene), which should be a minor product as per Saytzeff’s Ruleis, formed as the major product, which justifies Bredt’s Rule.
However there are many exception to Bredt’s Rule i.e., as the ring size of the bicyclic system increases, double bond can be introduced at the bridgehead with puckering of the rings. An example to this exception is Bicyclonene, which has a bridgehead double bond but is stable .In this compound the planarity of the pi-bond is accommodated by the puckering of the large ring.