Back channel negotiations are usually ones conducted in secret by lesser diplomats, often when the two (or more) countries involved claim publicly to have suspended relations. These are often arranged through the good offices of a neutral country. "Contours" are the delicate shapes and curves of something - in this case it refers to the details of an agreement.
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User_garyAccording to reports, the back channel struggled for three years, with meetings in third country capitals to thrash out the contours of a Kashmir solution.My paraphrase: