Thinking only about this Bikram Hot yoga posture as a compression will stop you from getting the total benefit from the posture. It is also a spine stretch, or extension. You want to round your spine up, arching your back (like an angry cat!) as well as compressing the front side of your body in a forward fold. 

The dialogue describes keeping your two heels in one line, and your teacher may also include the instruction to not cross your back heel over behind the front. The Archer hot yoga towel can support you in this placement by letting you line up your heels using the design to place your feet as they should be.  Whether you stand on the towel or across the towel to do this posture the lines will assist you. When you bring your fingers down to the floor they can be placed on the midline, your two heels aligned closely either side of that centre line. 

Figure 1. Standing Separate Leg Head to Knee Pose is practiced standing on the mat in some countries and stepping over or across the mat in others. Either stance is supported by The Archer Hot Yoga Towel. 

Often we bend the front knee bends as soon as we start to compress down in anticipation of the head not meeting the knee. Instead keep both legs tight (contract your quadriceps, let the hamstrings relax and stretch) and really pull your core in (remember this is a forward fold and requires an activated core to protect your lower spine). Contract your transverse abdominals, pulling your bellybutton up towards your spine as you round down. Keep curling inwards with your legs straight and then, once you have gone as far as your anatomy will allow, if your forehead and knee still don't meet, bend your knee (a little!!) out to meet your forehead. Bending your knee immediately means you will flatten the spine as you descent not round up. This means no compression front side. 

Keeping your hips aligned is so important here. It will keep your spine straight as you round it up to the ceiling and allow your hip ball and socket joint to relax and stretch. If your back heel wants to lift off the floor you might want to try a smaller step. The weight should be over your front foot, distributed evenly and with each four corners of your foot taking equal weight. Open your hands if you need to, use your fingertips to push into the floor and compress frontside more while rounding up. 

Alignment of your hips and placement of your feet will give you a solid foundation to start from. Stretch up first (spine expands) then compress inwards and down. Contract your quadriceps and core as you go down, delay bending your knee until you have finished compressing down and then bend it outward only as much as you need to. Use 80/20 breathing through your nose and shift your weight to your front foot. Don't worry too much about open hands, focus on keeping the hips aligned and compressing in and expanding up.