Leg raises can also be performed hanging onto an overhead bar. These are known as hanging leg raises and are more challenging than lying leg raises.
They can also be performed on other apparati such as dip bars and captain's chairs, which also involve the torso being suspended in the air, except that the stress through the arms is different.
This variation of leg raise allows the pelvis to freely rotate. It is generally the more difficult variation for the abdominal muscles due to having to support the pelvic weight as opposed to simply stabilizing its alignment.
Alongside this however, it is also potentially the easiest to use bad form on, because users may use a swinging motion to 'cheat' by building up momentum. If the abs are not properly engaged, the spine can easily hyperextend and go into anterior pelvic tilt. It should be going into posterior pelvic tilt if the movement is being done to target the rectus abdominis.
This movement can also be done with "ab slings" which hold the humeri in ~90 degrees of shoulder flexion. This allows to do a more traditional crunch (exercise) by bringing the knees up to touch the elbows. It is however possible to assist in this movement by using the lats and other muscles to perform shoulder extension.