This is Katie Garner's explanation:

Using earlier-developing muscle memory to fast-track individual letters/sounds in 2 weeks to 2 months requires allowing for the “follow-through” of motion, which is what causes the slight “shadow” schwa sound when singing the voiced consonants. The shadow schwa is a naturally occurring sound in speech that can be heard following any voiced consonants. (Say the word “Bob” out loud and you’ll hear a slight “uh” sound at the end. That is the shadow schwa.)

Alternatively, abruptly stopping or “cutting off the sound” disengages muscle memory, which would make it unreliable for automatic retrieval.

Additionally, research shows that unnaturally cutting off the sound can interfere with blending; whereas allowing a slight follow-through of sound actually makes it easier for eginning learners to blend sounds together.

To this end, some studies recommend using an “ihh” (short //i/sound) instead, stating that it’s less likely to be exaggerated. However, this would only work if kids didn’t know the short vowel sounds yet, which Secret Stories gives on Day 1. This, kids using SS would instantly recognize it as short /i/ and then try to spell it!

This is explained in more detail in the book and music download that come with the kit.

THE BOTTOM LINE…

What matters most is taking advantage of earlier-developing muscle memory to fast-track letter-sound mastery alongside the phonics Secrets, so as to GET KIDS READING. That is the goal, not practicing how to make individual letter sounds.