Bidirectional traceability needs to be implemented both 
forward and backward (i.e., from requirements to end 
products and from end product back to requirements).

When the requirements are managed well, traceability can be 
established from the source requirement to its lower level 
requirements and from the lower level requirements back to 
their source. Such bidirectional traceability helps 
determine that all source requirements have been completely 
addressed and that all lower level requirements can be 
traced to a valid source.

bidirectional traceablity:
Take the example of mercurey quality center in this you can 
link requirement to testcases and testcases to defects and 
vice versa. This helps nothing is missed in testing process.

its use to check the product
in two ways 
#requirement  to product.
#product to requirement.
requirement -> client side paper

or checking the product with client side paper or reruirement