The term "downstacking" refers when a player clears through garbage. It is one of the most important skills in multiplayer Tetris. Downstacking is also an element of Tetris that is rich in challenge and depth. Inefficiency in this area is often the Achilles' heel of newer players.
The primary goal in downstacking is to lower your surface height. Doing so prevents you from topping out. The goal is to use as few pieces per garbage row as possible. Unnecessary pieces are a source of waste. Becoming more efficient in this regard has a very real effect on your ability to win.
What lies below determines how to stack above. The location of each hole reveals important information as to how you should best place pieces above. The higher the hole, the more important it is to stack with it in mind. Simply put, try not to stack over holes. You must clear any upstack left above the hole before you can clear the hole itself. Clearing those upstack rows require additional pieces.
In this example, stacking over the bottom hole would cost you at least two pieces more than necessary.
Every cell of upstack aligned directly above a hole represents an additional line of upstack that later needs clearing. In the example below, the I-piece is the least desirable solution. It leaves behind three cells of residue directly over the 10th column's hole. Those three rows of upstack will cost you roughly eight additional pieces. The S- and J-pieces are much more appealing. They only leave one cell of residue over the 10th column's hole.
Placing the O-piece against the left wall makes it easier to resolve the upcoming holes on the right side.
In this case, the S- and L-pieces leave residue directly over the next hole.
The J-piece leaves nothing over the next hole, but does leave residue over the hole after that.
The T- and I-pieces do not leave residue over any holes. However, the T-piece is best since it clears two lines. Also, it results in either an I- or J-piece dependency. The I-piece results in an I-piece-only-dependency.
Examples of avoiding covering holes
Below, it is better to postpone the immediate line clear.
Managing Upstack Residue
It is possible to clear residue without using any extra pieces. Below, an L-piece sits directly over the bottom-most hole. However, this does not cost additional pieces. Its residue disappears as a natural consequence of clearing the holes above it.
This is why it is useful to spread residue evenly over the surface. It tends to result in more opportunities for clearing residue away "for free." At times, you can place pieces so that they do not cover holes. This can result in awkward surfaces. The strategy changes depending on how much upstack covers a hole. When there is a lot, then creating instabilities may not pay off. Chances are, you can clear off the upstack naturally before reaching the hole.
When dealing with a hole near the top, it is more worthwhile to forego stability. Competent downstackers know when to play it safe and when to take chances. It is a matter of judging which path best suits the end goal of minimizing overall pieces used per garbage lines cleared.
The first example results in a stabler surface. However, it is more valuable in this case to start clearing garbage while maintaining a "good enough" surface.
Here, a temporary gap in the surface is likely worth clearing down that garbage.
Finding opportunities to clear linesSingles and Doubles tend to be more efficient than Triples and Tetrises when clearing through upstack. Every piece you drop that doesn't clear a line, adds to the upstack. This deters future garbage-clearing. Keep a look out for ways of clearing lines with whatever pieces you have at your disposal.
Focusing on clearing garbage rows is usually more efficient than dealing with existing upstack rows. Each row of upstack requires 2.5 pieces. Yet, each row of downstack only requires 1/4th of a piece.
In the below example, both cases leave the same number of filled cells. In both cases, a line is cleared. However, the second case reveals access to the next hole. While it is good to minimize upstack, it is better to clear through garbage more directly in this case.
The jagged cornerThis shape was shown in the upstack section, but it poses even more danger when downstacking. The Z-piece will not resolve it.
Even the J-piece is better, since it will not require more pieces stacked on top of it.
Maintaining downstack optionsIt is usually good to direct upstack residue away from upcoming holes. This enables more options to clear holes later.
Attempt to keep the area around and above upcoming holes as clear as possible.
Finding alternativesIt can sometimes be risky to hold out for the "perfect" piece. Every piece that comes before the perfect one must be placed somewhere in the meantime. This can result in unnecessary upstack. Keep an eye out for alternative methods.
Typically, it is good practice to skim pieces in a way that lends well to skimming for other upcoming pieces.
Platform StackingVertical placements can be troublesome when they cover holes with more upstack than may be necessary. Sometimes less intuitive horizontal placements are the smartest way to proceed. The term "platforming" refers to deliberately allowing temporary gaps or holes to this end.
Lying lowWhen possible, avoid placing pieces above the highest occupied row. Doing so likely will require more pieces than had you found a spot within the confines of existing upstack. This may mean needing to create holes or gaps. A higher and neater surface frequently requires more pieces than the messy and low one.
In the below example, the messier method results in less upstack.
Below, incorporating the T-piece results in less overall upstack. This is the case despite both methods only clearing one line.
The first example shows superfluous stacking above the current highest occupied row. This creates a need for four extra rows of upstack in the upcoming future. These two pieces could have been used towards downstacking instead.