Compliance

Compliance is simply the opposite of stiffness. A steel jaw is not compliant: it goes exactly where it is told and stops only when something breaks. A rubber or flexible plastic finger is compliant: it gives way as it meets the object.

That give matters because robots are rarely told the exact shape of what they are holding. A stiff gripper that closes two millimetres too far will dent a package or shatter a shell. A compliant one absorbs the error in its own bending.

Soft gripper research exists mostly because rigid fingers assume the object is neither fragile nor squishy. Add compliance and that assumption goes away, at the cost of grip strength and placing accuracy.