The "new improved" sampler introduced in Corona 1.7 is the DMC sampler well-known from V-Ray.
Why was it added?
The most obvious reason for implementing the DMC sampler in Corona was that it simply delivers noise-free images faster than the previous solution. The difference is mostly visible in such features of the rendered image as antialiasing, depth of field, and motion blur.
It is also the first technological solution that has been ported from V-Ray to Corona Renderer.
How to enable it?
The DMC sampler is enabled by default in every newly created scene, so there is no need to manually enable it.
The setting is called "Random sampler" and can be found in the Development / Experimental stuff rollout. The DMC sampler is called "New improved" and the previous solution is called "Stratified". This setting should not be changed, unless it is explicitly requested by a member of the Corona Renderer team (e.g. on the forum on through a support channel).
Examples.
New improved sampler, 2 minutes:
Crop:
Old stratified sampler, 2 minutes:
Crop: