After a good inhale there are (to my mind) three ways to prevent the air from out.
1) By using the middle part of the tongue to create a seal (as taught in the course)
2) Use the very back of the tongue to create a seal (which is often mistaken for closing the throat)
3) By continuing to engage the inhale muscles.

Number 1 is what is taught and should be developed.
Number 2 should be avoided at all costs.
Number 3 should be treated with caution, has limited use, but can be used safely in certain situations.

Also a combination of 1 and 3 can used, particularly in soft delicate attacks.

Hope this is helpful. Remember, does it sound good? does it feel good? That’ll be your guide.