To enable beat by beat recording on the Garmin 920XT (or Garmin Fenix3 or Forerunner 620 or 735XT, 630 or most newer models) you need to do 2 things
1. Enable per second recording (NOT smart recording)
2. Open (this) zip file and extract the file “enable_hrv_settings_file.fit” directly to your 920XT in this folder: :\GARMIN\NEWFILES Sorted.
As of July 2016 this will NOT work PROPERLY with optical HR.
Now; how you plan to look at the data and/or use it in your training is another matter entirely 🙂 Maybe with Firstbeat ATHLETE software? Or Sporttracks using the Garmin FIT Importer plugin and/or Liverecording plugin? Here is a link for the Garmin Forerunner 920XT Review.