The rings on a loose ring bit are completely loose and can rotate freely.
This gives the rider more play as the rings shift with the change in rein pressure.
The oval elliptical link is basically a variant of the French link, with a rounded lozenge instead of a flat spatula joining the two halves of the bit.
The benefits are largely the same, with reduced nutcracker action, more even pressure over the bars, and independent control over the two sides of the mouth.
This encourages the horse to be lighter and lean on the bit less, as there is more mobility in the mouthpiece.