The combination of different interests, on the principle of “Claw me, I'll claw you.” Applied to mutual admiration criticism. One friend praises the literary work of another with the implied understanding of receiving from him in return as much as he gives. The mutual admirers are called “log-rollers.”
In the last decade of the nineteenth century, it was used politically to signify if A B will help C D to pass their measures through the House, then C D will return the same favour to A B.
Of course, the term is American. If you help me to make my clearance, I will help you to roll away the logs of yours.