I learned this teaching technique from Kim English (an American painter who's work is exquisite). The idea is to get to what's important, to capture the essence of the subject without getting caught up in detail. One learns to see what is essential very quickly. I find as I am blocking in a 15 minute pose, I am constantly noting what is of greatest interest in my subject so as my time winds down I will be able to prioritize what I want to include. The difference between this approach and a finished studio piece where there is infinite time to ponder these things is that the pressure of time running out really helps push the symbol oriented, judgmental left brain out of the equation.
It is a very difficult approach for some students to embrace, the resistance seems to come mostly from our natural tendency to be result rather than process oriented. Especially in painting, we seem to want all of our time spent to end in a finished piece we can hang on the wall. This is compounded when one gets paid for those pieces, but it can be quite freeing to embrace the idea that a lot of our time spent should be working on skills with NO immediate end result expected, just faith that the work we are doing now will help us shine brighter when it counts.