We have eight GHDs at the shop for a reason – the apparatus enables one of the top collection of accessory movements that we want you to incorporate into your training outside of your hour CrossFit class.
The GHD develops midline stability as well as awareness of movement. The exercises most commonly done on the GHD are the hip extension, the hip and back extension, and the GHD sit-up.
We want you, our athletes, making use of this apparatus in the 3-4 minutes before or after class. As a starting point, athletes should be able to perform 25 hip extensions, 25 back extensions, and 25 GHD sit-ups each in a row. Athletes should aim to hit each of these three movements one time every week. As a gauge, it takes 1-3 minutes to perform 25 of these repetitions – not a bad commitment!
During classes we use the phrase, “you can do GHD sit-ups (or hip extensions) in today’s workout if you have voluntarily been on the GHD in the past two weeks.” That means, if you’ve been doing your homework and maintenance on the GHD outside of class, you can do GHD sit-ups in the workout. If you haven’t been on the GHD consistently over the previous 2 weeks, we will scale to another movement such as an abmat sit-up or V-up. We don’t recommend you try to sneak around this requirement (see below), plus, the coaches know – we have a 6th sense for that kind of stuff.

We do this for two reasons, first, safety. The GHD is a fantastic, yet potent, training mechanism. Most workouts involve at least 75 repetitions.  We want athletes on the GHD only if they are staying acclimated to the exercise. More on this point below in italics. Second, it’s our way of keeping you on the GHD outside of class, pretty sneaky, huh?! Do you want the privilege of doing GHDs in the workout? Put in the time before and after class along the way.  
Not sure how to do a movement or need a tune up on the GHD? Just ask! Occasionally, we spend a week reviewing each of the movement types during class and we’ll be doing that soon.
Check out this article for more info! We’ve pasted an excerpt from the article to give you a little more background on why we don’t want people jumping on the GHD if they haven’t been putting in the time.  

Our favorite story along these same lines comes from Matt Weaver (www.speed101.com), arguably the world’s fastest human being. On top of being known for hitting 85 mph on a bike, Matt was crowned “King Sit-Up” in high school for completing 100 perfect-form body-slapping sit-ups in one minute. In one of his earlier visits to CrossFit Santa Cruz he found himself in a multi-station circuit with a group of CrossFit veterans that included twenty-one reps of GHD sit-ups with a full range of motion, hands reaching back to the floor. The workout left Matt sick in the immediate aftermath. 
This was a surprise for sure but nothing prepared Matt for what came the following morning: “I awoke later without the slightest ability to sit up. It was as if the six pack was totally gone, though all ribs remained. The curse had left me merely able to roll over and slither like a snake off the edge of the bed. From there I had to use my arms in humiliating ways to move about. I avoided being seen. A week passed, and I began to revitalize.” 
The worst was yet to come! In the wake of Matt’s being dethroned as King Sit-Up, his abs had swollen and distended markedly. This kid looked fat and sunburned where the week before he’d been ripped and lily white. As the swelling subsided, his scrotum grew and grew and grew. Matt’s father, John, is an ER doc, so he was consulted. He laughed himself to near seizure. (Our kind of doc.) Before the swelling stopped Matt’s scrotum had become the size of a small and very ugly cantaloupe.