Hardtack Stacking (乾パン積み) is a TD Attack opener which was invented by Sioyaki.
The first bag requires early O and Z or S pieces to build. The build rate is 54.44% with mirror, and 23.33% without.
The 2nd bag is built depending on which of J and L comes first. If J comes first, build the top setup. Otherwise, the bottom one. The OSZ ball is interchangeable, and by using all of the shapes, you can avoid softdropping.
Third Bag and Beyond
For J before L, you can go for a TST or a Fractal depending on which of J and Z come first in the 3rd bag.
For L before J, you can always go for a TSD as shown below, though freestyling is better if you can avoid making an I piece dependency.