I'll answer the Original Poster's question head on:
In relation to the description of vowels, the labels ғʀᴏɴᴛ and ʙᴀᴄᴋ notionally relate to which part of (the body of) the tongue is raised closest to the roof of the mouth: the 'front' or the 'back.' They don't relate terminologically to the retraction of the tongue root, and they don't relate to the tip or blade of the tongue.
Unfortunately, the named parts of the tongue do not correspond to intuition or common sense. When you look in the mirror your tongue looks kind of flat and thin. This is an illusion. Your tongue is really a big fat ball with a bit stuck on the front of it.
When we talk about the tongue, there is the front bit which we use for consonants. We can flap it about in all kinds of different ways. That's the blade of your tongue. It doesn't count for vowels in any way. The part of the tongue that we use for making vowels is the big ball of muscle behind that. It starts where the your tongue joins the floor or your mouth.
In the stylised diagram above, the blade of your tongue is represented by the small pink rectangular section. The square behind that represents the ball of your tongue. Now when we talk about "front" vowels and the "front" of your tongue raising, we are talking about the front of this back part of your tongue. So when you think of your tongue in normal everyday terms, this is actually about half way down your tongue as you look at it in the mirror. This is what raises towards the roof of your mouth when you make an [i] vowel. The blade of your tongue does nothing at all.
So the horizontal axis of a vowel quadrilateral shows whether it is the front of that bulky part of the tongue or the back of it that "is raising up". (The vertical axis represents the distance between the tongue and the roof of the mouth.)
That said, the production of vowels is much more complicated than the idealised and simplified descriptions relating to vowel labelling in terms of rounding, height and front/backness. And, especially with open vowels it is extraordinarily difficult, if it is possible at all, to finely manipulate the front or back vowel quality of the sound by consciously trying to change which bit of the tongue is most raised. It's far easier to hear examples of the sound and aim for those, or look at someone making an exaggerated articulation (preferably with no sound whatsoever) and try to produce the sound you think they are making. Another way is to produce the sounds down either edge of the vowel quadrilateral. So if aiming for [a], travel down the front vowels going from close to open. So go from [i] to [e] to [ɛ] and then continue lowering the jaw in the same way to [a].
Out of those different techniques, in my experience, by the far the most effective for non-high front or back vowels is the 'exaggerated' pronunciation. The best way of doing this is getting someone who can make the sound to pretend they are screaming the sound at the learner for a couple of seconds, but without making any noise at all. (Works beautifully.)