How to use the skills that Sword and Deadly Bullet bought from Yarugo?

Some students don't know how to use the skills bought by informants.

Please note that the gun icon in the lower right corner of my picture can be full of energy. When the energy is full, press the middle mouse button V to activate the skill.

Different weapons have different effects, for example, pistols can greatly shorten the cooling time of all skills in a certain period of time, gatling can cause ground explosion after overheating, and assault rifles can send out flame bombs.