Credit to Okey_Dokey for the original explanation
It can be not very clear for beginners to understand why some T-Spins are counted as T-Spin Minis while others aren't, and why some are not counted as T-Spins at all. Hopefully, this page clears things up.
When the T piece's last action was a rotation, it counts as a T-Spin if it meets certain conditions.
At least 3 of the 4 corners must be filled in the 3 × 3 slot where you place the T.
If not, it just counts as a regular line clear. For the example below, only 2 corners are filled, and therefore, it's not considered a T-Spin.
This rule dictates whether the line clear would be counted as a full T-Spin or a T-Spin Mini. A T-Spin Mini is like a "lesser" T-Spin—it sends next to no lines, although it keeps Back-to-Back. To be counted as a full T-Spin, the T piece must be facing 2 of the 3 or 4 filled corners on its final position.
The examples below are T-Spin Minis because the T pieces are only facing 1 of the filled corners.
This is where it gets a bit confusing. The 2-Corner rule is overridden if certain kicks are used, namely, the "TST" kick and the "Fin" kick. Both of them are the last kicks tested when rotating in those directions.
Line clears made with these kicks are always counted as full T-Spins even when the T piece would only face 1 of the 3 filled corners.
Now that we've covered the basics and the two exceptions, it would be a good time to reason how NTSDs are counted in the game. These are TSDs that involve kicking the T piece 2 rows down compared to traditional TSDs.
- The first one is an Iso-TSD. It satisfies both the 3-corner and 2-corner rules, so it's counted as a TSD.
- The second one is a Neo-TSD. It doesn't satisfy the 2-corner rule, so it's counted as a T-Spin Mini, but since it clears 2 lines, it's a T-Spin Mini Double.
- The third one is a Fin-TSD. Although it doesn't satisfy the 2-corner rule, as mentioned above, it counts as an exception since it uses the "Fin" kick. It's counted as a TSD.
A lot of players are probably unfamiliar with T-Spin Mini Doubles. Doing a Neo-TSD is one way to achieve it. There are 2 more cases where you can "downgrade" a regular T-Spin Double into a T-Spin Mini Double.
- First example: You do a "TST" kick, so it counts as a full TSD at first. However, by further rotating it, you downgrade it into a Mini since only 1 corner is filled, and you haven't used the "TST" kick to put it back there.
- Same as above. You do a TST kick, which makes it count as a full TSD, but the subsequent "Neo" kick downgrades it.