Acqua and Knight Leader continue their fateful showdown in the forest. Princess Carissa reveals a devastating new technique using Curtana Original. When Kamijo and Index's lives are endangered, an unexpected ally comes to their rescue.
It's something of a safety 
device put in place.
On peut dire qu’il s’agit
d’un dispositif de sécurité.
In pratica, funge da limitatore di sicurezza.
Иными словами,
предохранительное устройство.