Now set the safety rails so that they're about even with your mid-lower back.

Set a bar on the rails, braced up against the uprights. Hang from the cross-beam again and note where the bar hits you. It should hit a few inches up from your hips, right in the small of your back. Adjust as needed.

Because the bar is just being used as a brace during the exercise (not for resting bodyweight on), you shouldn't need any padding on the bar. The bulk of your weight is supported by your hands in the hang.

(Note, this exercise setup will only work if your hands are set slightly BEHIND the bar, which is why I recommend gripping on the actual cross-beam of the rack. You'll notice I have a black chin-up bar on my rack towards the front. This doesn't work for this exercise because it's set too far in front of the end of the rack. When the lower-back bar braces up against the rack uprights, my hands are in FRONT of the bar with that setup and I don't get the bracing/pivot effect we're looking for).

Now that we're set, the fun begins...

While hanging, bend your hips to about 135 degrees - i.e. so that they're somewhat bent but still relatively open. This is not part of the exercise...it's still part of the setup for your body.

Now LOCK your hips in that position. They should not move much (or ideally at all) when performing the exercise.

Perform a "lower body" raise.

The bar in your back creates a new pivot point that simply isn't physically possible when you're free hanging. This is why the hip flexors HAVE to get involved to perform the normal movement.

Use the bar as the pivot and raise your entire lower body (from that mid-lower-back point down) as one unit..essentially you want to "lock down" the lower body, from your hips to your feet, in position then raise the whole mess up at once, flexing from the lower back.

This is the KEY to the exercise.

Come all the way up until your upper body is leaning back at an angle. Try to keep your hips locked in that same position and same angle. You don't need to bring the knees up further towards your chest.

And even if you're normally strong with hanging leg or knee raises, this will show you how much the hip flexors get involved even when you're trying to keep them out of the movement.

When you watch the video, even though it actually LOOKS like I'm opening up the hips at the bottom, what I'm TRYING to do is get more extension of the spine at the bottom, wrapping my lower back around the bar. It's tough to maintain the same exact hip angle...but it's something you want to strive for.

The movement is truly a lower ab exercise, locking in the upper body and the lower body and using the rectus abdominis to draw the hips and rib cage closer together with ZERO hip flexor assistance.

Aim to get as many reps as you can with good form. Once form breaks, end the set.

You're also going to notice a VERY strong isometric stretch in your lats when you perform this exercise, similar to a Front Lever exercise. It's as though your lats are trying to perform a pullover with the weight of your entire lower body. The abs are the only muscles actually contracting and moving but the lats do contribute isometrically.

Bottom line, this a really challenging, incredibly effective lower-ab exercise. If you've got strong abs and are looking to truly hit the lower abs without the hip flexors, give this one a shot.

I definitely recommend watching the video for this exercise to get an idea of how the setup works and how the movement flows.