While it may look that the heels are off the ground in this picture, this is done with the feet flat on the ground.
This exercise is particularly beneficial for those with tight hamstrings. Remember to start with the feet aligned and hip width apart.
It doesn't really matter how far down your hands reach or even if your legs are straight (yet) as the aim of this exercise/Asana/stretch is to improve the flexibility in your legs and spine.
If full forward fold is too intense, try coming down half way and supporting your upper body weight with your hands on your thighs.
If Pilates is your exercise of choice rather than Yoga you will recognise this as the bottom of a roll down - if practising at home, pause at the bottom of your roll down to enjoy the stretch you get in full forward fold.
Please do be cautious if you a back injury or suffer with Osteoporosis as this exercise may not be suitable for you