Here's another short clip from the Spring 2009 workshop in Tempe, Arizona of my Sifu (Master Sam F.S. Chin) discussing using attention to manifest fullness to the point of contact.

Keep in mind that when he uses the term "peng" that I-Liq Chuan has it's own concepts regarding peng which in many cases vary drastically from some arts like tai chi chuan.

The fullness is one of the "six physical principles" of I-Liq Chuan (technically it's defined as the internal and external spheres of offense and defense). Here, Master Chin mentions the concept of highest point, which is part of the concept of fullness. The key idea behind the highest point is that no matter where you touch a sphere, it's equi-distant from the sphere's center of mass (i.e. it's the highest point on the surface of the sphere).

The concept is that by developing our concentration though practice we can then recognize the size of the sphere according to the point of contact with the opponent. If you aren't mindful of the size of the sphere, you won't be able to manifest the fullness.