The hands are held high, not jaw height, but temple high. Holding it at the jaw is great if you have gloves on. But it won't stop anything if you were fighting empty hand. Elbows are pulled close to the chest to cover solar plexus and ribs.
The jab punch skill starts at 8th Kyu or when you're learning Dan-gun for our school. The jab is not the same centreline extension as the basic lunge punch. The jab gets your elbow raised and utilises shoulder rotation to generate power. The cross requires hip and shoulder rotation in order to strike at the right reach and distance.
After about 20m of drilling this line, the step that we were taking was lengthened in order to make this jab cross effective for real engagements. The step drag routine was discussed - the front foot sliding forward across the wooden floor was too easy. We introduced another thought - that of sliding the foot forward on uneven territory. So the front foot is pushed forward with outer blade of the foot pointing straight ahead. In normal walking, the inside of the foot points forward. We point our foot inward in order to use leg muscles to drive ourselves forward. For uneven ground, we also need to pick the toes up - like how the nose of the plane rises in order that the wheels hit the ground correctly.
Taking the longer step forward you see an immediate problem - most people end up weighting their COG on their back foot. Now - if you wanted to intimidate your opponent you need to make him think that you are going to smack him in the face with your fists. Doesn't matter if you're going to try and break his kneecaps with a kick - you must be weighted forward in order to use your hands.
Taekwondo practitioners listen up - in a combat situation you need to launch your kicks whilst looking as though you are going to punch your opponent out!!! That means more weight on your front foot and back foot poised to push you ahead. Not leaning backwards and favouring your speedy roundhouse kick or your back kick to save the day!
Also for god's sake ... your hands need to be up and your head needs to be down and tucked in! The last thing you want to do is announce to the world that you're a taekwondo look-at-me-just-like-what-you-saw-at-the-Olympics black belt. It is not going to save you. (Especially if your opponent is a referee or some sporting judge).
Once breathing starts being rhythmic we changed the exercise and got people to do the drills backward. This is for when an aggressor is in your face, pushes you backward and comes at you. So students get a target stuck right at their faces and on my count step back and fire off two shots.
Continuation post at Do San Defending Against Straight Arm Blast