Yes and no.
Yes, the explosive lift stresses the upper back muscles really well. But if you have a lot of weight on the bar, and the rest of your body can handle it, and you start from pins or blocks in order to spare your lower back some and allow greater load, you'll hit those muscles without the explosive movement. A sufficient load would be one that will rip your shoulders apart if the upper back muscles don't resist.
And no, don't execute the third pull of the snatch (don't get under the bar.) The explosive version of this movement is all about getting the bar up high, with your scapulae going from fully extended to fully retracted and back to fully extended (when you bring the bar to a halt at mid-thigh) under the greatest possible tension.
Going to make a video now.