To explain arrows thoery,It is realted to maths.My fav subject.

Consider 5 attacks and 5 defenders.
Attacking rate of attacker before forward arrow=n
Attacking ratw of attacker after forward arrow=n+1
Defending rate of defender before backward arrow=y
Defending rate of defender after backward arrow=y+1

Case 1;Players have no arrow.
Attacking rate before forward arrow=Defending rate before backward arrow.

n=y keep it in mind.(P)

Case 2:When all players have arrow.
Attacking rate of attacker after forward arrow=Defending rate of defender after backward arrow.

n+1=y+1 or n=y+1-1
So result n=y. As it is equal to above result,hence proved.

Result=There is no net change in player when there is no arrow or all arrows.
Bro now its clear?