Imagine that you're doing your favorite activitylet's say, sailing. You're skimming along the waves, when suddenly the breeze freshens. You hike out to compensate, leaning back into the wind to keep the boat upright. A wave splashes your face. You shake your head and trim the main sheet for more speed. You are entirely focused on the movements of your body, the water rushing past, and keeping the boat right side up.
You're really flying now, just on the edge of control. You're so fully immersed in this activity, there's no room left in your awareness for distractions. Otherwise, you might catch a wave and capsize. You're having so much fun that you want this moment to last forever. Mihaly Csikszentmihalyi calls these exceptional moments flow experiences.  Flow can occur in practically any activity, including browsing the web.
This "optimal experience"  is "intrinsically enjoyable."  Time seems to stand still, and we lose our sense of self. We feel playful and are willing to try (and presumably buy) new things. Although flow can occur anywhere , certain activities like rock climbing, performing surgery, chess, and sailing lend themselves to this optimal state of focused attention. Responsive, well-designed web sites can also induce flow in their users.