Sitting with your legs straight out forces you to develop a considerable amount of hamstring flexibility, along with the requisite core tension to remain upright.
As you press up, keep the implement stacked over your shoulder and hips until the finished position.
You multitaskers out there will love this movement as it allows you to work the rectus abdominis, the obliques, and the serratus at the same time. Too bad the rest of your life isn't as efficient.