"Secret Grade" is a hidden grading mechanism in the TGM series that rewards creating a ">" shape by leaving holes in the stack.
Basic Strategy
Creating the correct holes in the stack is easier to accomplish when a "sawtooth" or "stairs" structure is maintained in the direction of the diagonal you are creating. By maintaining this structure, holes can be covered with others to finish holes and create the foundation for new ones. S, Z, and T are very useful for creating the sawtooth structure or covering holes, although any piece can be used to create useful structures.
Line clears increment the line counter and can prevent progress. For these reasons, line clears are generally avoided. However, line clears can be useful or necessary in a variety of situations. Line clears are unavoidable when creating the first holes at either wall. If a line clear is necessary, holes can be built on top of the lines to be cleared and "dropped" into place.
Maintaining an even spread of pieces throughout the stack is very important. Uneven stacking can hinder the completion of holes and reduce placement choices near the top of the stack. Placement choices are limited near the top of the stack, so care should be taken to maintain an even stack without obstructions. If it appears that the stack is growing unstable, the best strategy is to clear lines or backtrack until a stable approach can be made.
Reaching the right wall is like starting over again; a line clear structure is needed to create the first few holes. Structures going to the right wall can be reused when returning to the left, but the pieces used must be flipped. For example, structures that used L before reaching the wall now use J on the return trip.
Starts
Starting the structure can be efficiently started with any piece but O. At least one line clear is required when using any other piece, but using O to create the first hole necessitates two.
Starting line clears can also be used in the body of the building process. Try to use structures like these whenever a line clear appears to be necessary.
Other Line Clears
Some line clears can be useful in creating holes, but have little or no use as starting structures.
Covers
In the case that you have an open imperial cross or T shape, fill it with S/Z finish a hole and start another.
Cover holes with S/Z to create the foundation for new holes.
Place T next to this foundation to complete the hole and start a new one.
Covering with I can also be effective, but be wary when using L, J, or O; covering with these pieces can create unwanted hanging points or fill the space where you intend to form a hole.
Hangs
Hanging points are structures that impede the direct creation of holes; pieces must instead be placed in so they are "hanging" from this structure in order to create a hole. Using O, L, or J to cover a hole have the highest potential for creating hanging points. A line clear may be necessary to continue, but sometimes "hanging" structures can be useful in quickly creating multiple holes with few pieces.
If the structure is 3 units tall, only L/J can be hung from the structure and complete a hole, making a line clear the best strategy.
If the structure is 2 units tall, all pieces but I can be hung from the structure to complete a hole. A few pieces will maintain the 2 unit structure, while a few will necessitate a line clear. T can successfully end the hanging structure without requiring a line clear.
In this diagram, O starts the structure and J and S maintain it. Z and L could have been used to end the structure by clearing a line, but T caps off the structure without a clear.
Finishes
The safest way to finish the pattern is to cover the sawtooth at the top with a Z, simultaneously finishing holes 17, 18, and 19.
Using a Z allows the pattern to be prepared for completion while row 18 is still mostly empty. Although the structure can be completed by finishing hole 18 first, the strategies for completing hole 19 at this point are extremely risky.
