There are 3 stages with regard to correct positioning of the tongue to develop correct open throat technique.
When starting out, the first is to keep the tongue flat and to open the back of the throat as much as possible.
As you get comfortable with this you will want to practice having the tongue "concave" to create even more space in the back of the throat (so you don "choke off" the air in the back especially when ascending scales).
The final stage is when you are so comfortable keeping the back of the throat open, the tongue can protrude from out of the back of the throat (like a lizard) not only creating more space but allowing for greater freedom in singing different vowel phrases within the shape of the vocal tract.
It also enables you to use your tongue for consonant sounds instead of closing down the jaw with causes strictures in the back of the throat.
But the frirst two stages must be mastered first or it is doubtful you will "embed" open throat subconsciously allowing for this final stage