When blowing into the long plastic bag, air from the surrounding room rushes into the fast-moving, low-pressure stream, increasing the volume of air entering the bag.  As a result, one strong puff into the bag picks up enough surrounding air to fill the bag, while directly blowing into it—as you would a paper sack to pop—requires 20-30 large puffs.

1.  Start out by unfolding the bag flat on the table.  Gather up the open end of the bag in your hand and directly blow into it.  Not much happens.
2.  Flatten the bag again, spread out the opening in a circle, and with both hands hold the opening a little ways up off the table to get a smooth line for air to enter down the length of the bag.  From about 10cm away, blow a strong puff of air into the bag.
3.  It should completely inflate.