Modify a readily available retractable reel (badge holder) so it has two operating modes. Use a switch or lever or something mechanical to change between operating modes from #1 to #2 and back.
Mode #1. Regular rectraction, pull out the string, let it go and it retracts.
Action: Flip or twist a switch to change mode.
Mode #2 Pull out the string to about 75% of full length. Then release and it stays in that position with the string loose. Pull the string again and it fully retracts.
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