A Keshin is an ability to show someone's spiritual energy. It is also capable of using Hissatsu Techniques.
The Keshin energy is mostly invisible for the others' eyes. When it is mastered, it becomes a visible form for the user's himself and for the others. It also reduces the stamina of the user greatly which can cause that the user is tired after using his Keshin.
A Keshin Battle is when two Keshin will clash into each other.
First the two Keshin along with their users ran to each other, choosing to either block the attack of the other Keshin or to attack the other Keshin head on. When that it chosen, the two Keshin will clash into each other and the Keshin with the greater attack or defense will take the win and has the ball.
A Keshin Hissatsu is a way to perform a hissatsu along with the Keshin of its user. Keshin Hissatsu is only performable with a Keshin and thus unable to use it without a Keshin. The attack is mostly higher than a normal hissatsu and it also uses KP, the TP bar of the Keshin, instead of the using the TP from the user. It mostly consumes quite a lot of KP. Like a hissatsu, the Keshin Hissatsu won't be able to use if the Keshin doesn't have enough KP.
Keshin Armed is an ability to "wear" your Keshin as an armor instead of the spirit form.
Keshin Armed is only capable when the user has a Keshin that can perform Keshin Armed. The KP bar will drop on the speed the user's stamina stat has. While being in Keshin Armed form, the user won't use up any TP while using hissatsu techniques but will use the KP from the Keshin. The hissatsu will be a lot stronger when playing soccer, making them more powerful, however the user's GP will drop after using Keshin Armed; mostly only the half of the GP is left.