I think there might be two issues involved in your post, correct me if I’m wrong:
1. Using the tongue to block the air and learning to release the air Passively simply by removing the tongue from behind the aperture/teeth/roof of the mouth, etc. and letting the air escape, as Greg demonstrates, etc.
2. Using the tongue to play legato or soft musical passages
I believe those are really separate things, but I could be wrong. Others or Greg may chime in with a different point of view.
For me, when I play legato, I kind of think of more of a “La La La La” sensation…I don’t think that’s literally what’s going on, but that’s kind of the thought. It’s probably more of a “da da da da”… (I won’t quote any song lyrics…don’t you just love the Police?).
I believe the tongue is simply rising into the air column and slightly interrupting it, it may not be completely stopping it, when playing legato / softer passages. I strive for keeping the air moving through the phrase and only interrupting the air as little as possible with the tongue to achieve the desired articulation.
Experimenting with the different articulations, as Greg demonstrates in the course, should help a bit. Even working on double/triple tonguing could help a bit; while double and triple tonguing are still typically crisp/staccato, using a different part of the tongue to articulate could help you learn to use a different part of the tongue that could help with legato.
Another thing I like to do is to alternate while doing Clarke I between Slurred (which is great/important for feeling the flow of the air and balancing Shape and Air, etc.) and different articulations (Single Tongue staccato, Single Tongue Legato, Double Tongue, Triple Tongue, etc.).
I need to do more of that myself, my technique has not been what it once was as I’ve been focused more on other things lately.
Lastly, alternating between Breath Attack and/or a “Pu” attack (as Greg demonstrates) and Legato on an exercise or musical passage could prove to be helpful.
I have found it useful and important to separate playing the note into the following steps:
1. Obtaining the pitch(es) with a breath attack or slurred on Passive Air.
2. Articulating the pitch(es) on Passive Air.
3. Articulating the pitch(es) with Active Air.
Depending on how high or low the pitches are, you may have to alternate the dynamics a bit from what is written in order to use Passive Air, but I have found that to be very important for me.
That’s my $.02 FWIW, hope that helps.