Its a fib. convergence, its a point where there is more than one reason to get into the market. From the A,B, you would look for smaller a,b, spread your fib. from A to B then from a to b. The convergence would be anywhere on the trendline where fib. retracements from A,B and a,b meet, preferbly a .382/.618, forming a C,c convergence.
See pic below.
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