Each time a unit is released it will slowly fill up the circle icon next to the blue boat.

Once the circle is full,  you can release a ‘vehicle’ unit which will give you an extra advantage in the battlefield.

To release a vehicle unit, you need to release their finger form tap and hold on the blue boat.

You should be able to see a different vehicle unit on the battlefield if you successfully release the vehicle.

You can equip a maximum of 3 different vehicle units at each gameplay, these units are alternatively released in order of their priority which you can set in the Units Upgrade section.