what happens when concurrent index create

i read documents,i find it that concurrent index create don't lock write.but
need scan table twice.it explain is following as:
   It scans the table once to initially build the index, then makes a second
pass to look for things added after the first pass.

please explain 1. what happens  when concurrent index create .tks.
                   2.how to trace a backend what do somethings internal.for
example oracle 10046 trace.

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