As in a previous question on Glossika's "phonics" line, this is IPA-inspired, but not strictly speaking IPA itself.
A comment above has provided the answer: it represents what could also be transcribed as [iə̯ŋ], where the schwa /ə/ is not syllabic, but an off-glide. Note that the superscript notation for /ᵊ/ (and other superscript vowels) is also possible in the standard for IPA as a glide.
This excrescent schwa is relatively well-known feature in certain varieties of Chinese, especially when there are both rising and falling diphthongs. Although I can only hazard a guess at the writer's aim, the notation does clearly distinguish Standard Taiwanese Mandarin (where 冰 has a monophthong) from Taiwanese Hokkien (where 冰 has a noticeable offglide, a feature it shares with colloquial Beijing Mandarin).
It also makes it obvious to learners; many a learner of Mandarin, when confronted with the highly diphthongised sounds of -ing in Northern Mandarin, fail to recognise it as -ing. This notation also forces learners to be careful of what the nucleus of the vowel is, as that has a bearing on the way tone is carried.