BT Cannon (BT砲)
BT Cannon is a TSD → TST setup devised by TCM-β. The "B" of BT comes from TCM-β's name. BT Cannon is essentially a DT Cannon with the TSD part (marked sky blue below) shifted 1 column.
BT Cannon as an Opener
The BT Cannon → C-Spin → Perfect Clear loop created by 26_nameless is a popular way to use BT Cannon as an opener. While the first T is used for setup, the subsequent 4 Ts are used for a TSD, TST, TST, and TSD which makes this loop efficient in terms of attack.
1st Bag
2nd Bag
3rd bag
The loop divides into 2 paths on the 3rd bag.
Path 1
Path 2
4th bag
Path 1
The first and second patterns are especially important.
Path 2
The second (and third which makes the same shape) pattern is especially important. Note how it makes the same shape as the second pattern shown on Path 1.
Tip
It is possible to spin your S in by rotating CCW when making this setup.
5th bag
Credit to Tamihodo for the organized perfect clear solutions
The perfect clear solutions with SZI on the right part are the most often used and the easiest ones to memorize. All perfect clear chances shown here are based on the assumption that you did the TSD.
1-1 (99.72%)
This pattern has a near 100% chance of PC if you can do the TSD.
Overall chance is 99.96%. Solutions
1-2 (100%)
This pattern has a 100% chance of PC if you can do the TSD.
Overall chance is 93.33%. Solutions
1-3 (93.06%)
Overall chance is 94.01%. Solutions
1-4 (97.78%)
Overall chance is 87.94%. Solutions
1-5 (70%)
Overall chance is 69.60%. Solutions
2-1 (95.83%)
Overall chance is 98.97%. Solutions
2-2 (100%)
This pattern has a 100% chance of PC if you can do the TSD.
Overall chance is 93.33%. Solutions
2-3 (91.11%)
Overall chance is 75.04%. Solutions
2-4 (89.17%)
Overall chance is 81.59%. Solutions
2-5 (97.78%)
Overall chance is 91.94%. Solutions
STSD / Trinity
When a perfect clear is not possible, you can build an STSD/Trinity instead on your 5th bag.
Mid-game BT
BT Cannons are rarely used mid-game because of a couple of reasons. First, it is hard to spot opportunities because it is one of the more complex setups. Because it has a complex shape, it also generally uses more pieces during setup, which means a higher chance of using a T on your stack, which would defeat the purpose of building this.
While having these drawbacks, BT Cannon does not offer an overwhelming benefit compared to other comparable setups such as DT Cannon or Triple Double which are much easier to spot and build. For these reasons, it is hard to spot other players using it mid-game besides building it in specific field situations.
Building BT Mid-game
Try to spot the above shape where you have a TSD slot and a row with an offset hole underneath (marked green on the first picture), and then build the yellow part shown below. There's just a limited amount of ways to make the right-side part mid-game while maintaining B2B and conserving your T mino. This is another limitation of BT.
Examples:
BTD
Because BT Cannon leaves a TST shape after the TSD, it means that you can build a shape that leaves a TD after the TSD, just like DTD. BTDs are usually built with the help of garbage holes.
BT STSD
A rarely used setup that looks similar to BT Cannon. However, this setup forms an STSD after the first TSD. Like DT STSD, this setup yields 3 TSDs.
The only difference between this setup and BT Cannon is the 1 block marked purple:
The easiest way to recognize opportunities and create BT STSDs would be to convert them from Sky Prop TSDs.
Examples:
See Also / References
See this summary table created by tony825077 to see a more complete list including less frequently used 4th bag and 5th bag perfect clear patterns.