In a glute-ham bench or Roman chair, lean back like you would to begin a sit-up and hold yourself with the abs tight and slightly crunched with the trunk approximately horizontal. Be careful not to allow your lower back to hyperextend by ensuring maximal ab and glute tension. Hold this position for the prescribed time.
 
Notes
Hold your arms at your sides for the easiest variation and move them progressively farther toward the shoulders and eventually extended back overhead to increase the difficulty. Weight can also be held on the chest if more resistance is needed.  
 
Programming
GHD sit-up holds can be done for 3-6 sets of 10-30+ seconds. Generally if you’re able to hold the position correctly for 45+ seconds, it’s advisable to add a little weight and build back up to 30-45 seconds again.
 
Variations
The GHD sit-up hold can be done with or without additional weight, or in a rotated position.