U-cups are lip seals and are primarily intended for dynamic applications. Fluid or air pressure forces the sealing lips into contact with the surface to be sealed against.
Polyseals are used on the piston cap on the lower end of MWD systems. The materials stocked by Hi-Tech Seals provide excellent sealability for this crucial part.
When using an loader or energizer (commonly an O-ring) you turn the U-cup into a squeeze seal that provides high sealing force at low pressure. As the system pressure increases, the lip loading also increases, automatically compensating for the higher pressure while maintaining a positive seal.