The augmented limb leads promise a stronger signal than the Einthoven leads by connecting two limbs together and measuring them against the other limb. They complement the Einthoven I,II,III by having their vectors exactly between the Einthoven angles. However, nowadays the slight gain in amplitude is no longer of relevance but instead they make it easier to estimate visually the electrical angle of the heart.
In modern ECG machines none of these leads are actually measured but calculated using the Einthoven leads:
Note that we don't mention N/GND in this clip but it is still connected to the right leg as shown.
Here is the ECG measurement data recorded in this clip.