This movement was inspired by multi-tasker Christian Thibaudeau,
who likes to save time by doing multiple things at once. One day,
while simultaneously urinating and brushing his teeth, Christian
had a eurekamoment:

Why can't I work my rectus abdominus, my obliques,
and my serratus at the same time?

The Serratus Crunch was born the very next day.

Strengthening the serratus will help reduce the risk of shoulder
injury and will actually increase your bench pressing strength by
stabilizing the shoulder blades. Building your abs, of course, will
keep women mesmerized as they trace their fingers along the deep
nooks and crannies of your abdominal crevices, you big Wolverine,
you.

The key point is to keep the arms perpendicular to the floor at
all times. As you "crunch" your torso up, you should
reach for the ceiling and try to bring the hands as high as
possible. Hold that position for 2-3 seconds. If it's too
easy, try it with your feet off the floor.

Now get to the gym and multi-task.