Q
When I set Counterattack
with 50 HP or less, is it put in the power charger after its effect takes effect, or immediately?
A
In this case, it's put in the power charger immediately without its effect taking effect.
It doesn't give a timing such as "at the end of the turn", so if your HP is 50 or less it's put in the power charger "immediately".
If it's in set zone C and battle damage brings your HP to 50 or less, it's put in the power charger as soon as HP has been resolved.
If you newly set it with your HP already 50 or less, it's put in the power charger immediately at the area enchant swap (before its effect takes effect).