"Secret Grade" is a hidden grading mechanism in the TGM series that rewards creating a ">" shape by leaving holes in the stack.
Creating the correct holes in the stack is easier to accomplish when a sawtooth or stair 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. O, L, and J have the potential to impede directly creating holes and requiring that pieces a placed so they are "hanging" off of the structure in place. A line clear may be necessary, but sometimes "hanging" structures can be useful in quickly creating multiple holes with few pieces.
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. Any piece can be used to create the first hole 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.
(Basic covers, imperial cross + S/Z, etc.)
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.
(row 18 Basic Z, row 18 risky T, and extremely lucky row 19 builds)