The "Hinge Test"

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To check for compatibility between leader segments

After you have tied in one or more sections of material to lengthen or repair a leader, this simple test will reveal if the materials are well-matched and likely to provide a smooth turnover. 

If the leader "hinges" when you do this, it suggests that the transition between two segments is too radical: i.e., one section is too thin or weak to support its connected partner and suggests the union will not "hold up" when cast.

If this "hinging" occurs, you should remove the offending segment(s) and replace it by tying on the next heavier (larger diameter) material.  This will strengthen the connection and provide a smoother turnover.  If you ignore this disparity, your casts will not turnover your fly easily no matter how hard you try.