Immaterial and Missing Power/Spell Card
Spell cards are what you would consider the "super moves" in this game. Patterned after shooting games where bosses go into various preset patterns at certain life totals, spell cards are what these patterns are called in the Touhou universe.
When you select your character, you are given the choice to select your spell cards. Each character has a total of six spell cards: three for your first lifebar, and three for your second. The second spell card is not used until you have lost a round, which is shown by your lifebar turning red and the kanji above your spirit meter changing. The second spell card is usually a powered-up version of the respective first spell card, with exceptions such as Patchouli who has a totally different moveset in her second spell card selection. You can blind your spell card selection by pressing D before selecting, and unblind it by pressing D again. Unfortunately, since there is usually only one optimal spell card for a given character and given how spell cards do not affect pre-declaration gameplay, the metagaming aspect of blind-selection is almost irrelevant.
During the course of the game, you will accrue levels in which to use your spell card. You can only have a maximum of 9 levels, and levels do not carry over in between rounds. In order to use a spell card, you must first "declare" by pressing 22D. This can only be done when you have at least 1 level of meter, which will net you a certain amount of time in which you can use the spell card you selected. You have approximately 16 seconds with 1 level, and an additional 8 seconds with each subsequent level; thus, at level 9 declaration (with 9999 ticks on the timer) you have approximately 80 seconds to do spell cards, which are all done with 236D (with certain characters having exceptions that are performed by 214D in addition to 236D for different effects).
Declaring will have a number of effects. First, before you even declare, there is a somewhat lengthy startup animation during which you are wholly vulnerable. Advanced players will know when you will need to declare and will bombard you with attacks so that you do not have a chance to let the animation proceed; the safest way to declare is always after a successful guard crush or a knockdown, whether it's after a combo or a bomb. Once you are declared, you have about 2 seconds of invincible time during which your lifebar will partially replenish. Note that all damage done in the game leaves a trailing red portion in your lifebar; this portion can be regenerated once you declare. Once your "spell trance" runs out, you can no longer gain level or declare again until the next round (if there is one).
Keep in mind that while you have no recovery after the declaration animation (and it is fairly ambiguous when you actually recover), your opponent is free to set up whatever attacks they wish around you in preparation. Also, depending on the character you are facing, you may wish to declare at different times. Characters such as Suika and Yuyuko are heavy hitters so you will want to declare earlier so you don't lose the chance to do so if you get randomly hit. On the other hand, characters such as Reimu and Meirin are fairly low damage-wise so you can probably stretch your time out before declaring a bit further (though it can be argued that the momentum these characters generate can be just as deadly as those who hit hard).