reftex-index-follow-mode

reftex-index-follow-mode is a variable defined in `reftex-vars.el'.
Its value is

nil

Documentation:
Non-nil means, point in *Index* buffer will cause other window to follow.
The other window will show the corresponding part of the document.
This flag can be toggled from within the *Index* buffer with the `f' key.

You can customize this variable.