Immaterial and Missing Power/Stagger system
Staggering is an addition part of a character that comes to light when the opponent is guard crushed. Simply put, when you have no spirit, you cannot use autoguard anymore, and certain attacks will place you in a staggered state if you block them incorrectly. This is move-specific, so it is important to learn your character and know which moves can induce stagger. Some characters can deal major damage during guard crush simply because their high/low stagger game is very strong, allowing them to continually land hits during stagger hitstun and deal chip damage when blocked normally.
A staggered state is what occurs when you block an attack capable of staggering the incorrect way, such as blocking Sakuya's 5B low. Normally, when you have no spirit and block the wrong way, your character will be hit normally as if they had never blocked the move. When you block a staggered move the wrong way, you will be damaged and be placed into special stagger hitstun. Subsequent hits blocked incorrectly will prolong that blockstun and actually count as a combo on the combo counter. However, during this hitstun you are still allowed to adjust your block, so any followup attacks that are blocked correctly will not hit you. Hits that stagger you also do a bit less damage than normal. Lastly, one very important thing to remember: stagger hits cannot kill an opponent when they are at zero life.
For example, let's say Sakuya guard breaks Reimu in the corner. Now Reimu has to defend against Sakuya's multiple high/low options without the privilege of autoguard. Sakuya goes in for a 5B and Reimu blocks low, placing her in a stagger hitstun. Walking forward, Sakuya does 5B again, and Reimu blocks low again. Trying to press the advantage, Sakuya chains the 5B into a 6A; however, because 6A is a mid hit, Reimu is able to block it successfully, ending the stagger combo. Finally figuring things out, Reimu decides to try and block high, but Sakuya goes for a 2B 6A. Since 2B doesn't stagger, both it and the 6A connect for a 2 hit combo.
Since stagger moves vary from character to character, their individual move attributes must be taken into consideration along with their other strengths. Reimu and Marisa both have very strong stagger games in the corner; comparatively, Patchouli has no stagger moves at all and thus has no stagger game to speak of, while Yuyuko has only 6A, 5B, and 66B to stagger with, both of which are disadvantageous to use. Knowing this facet of your character is an important addition to what you can accomplish once you achieve a guard crush.