Final answer:
La mise en plis is the French technique that uses fingerwaves and flat pin curls to create retro hairstyles with defined waves or curls.
Step-by-step explanation:
The technique that uses fingerwaves and flat pin curls is called La mise en plis in French. This technique is commonly used in hairstyling to create retro hairstyles with defined waves or curls. Fingerwaves are created by using the fingers and a comb to shape the hair into S-shaped waves, while flat pin curls are achieved by wrapping sections of hair around the fingers and securing them with bobby pins.