The pause created by the dash in line 2 emphasizes what?

Dickinson's prolific use of the dash always raises questions for the reader. What do we make of them? Aside from giving her poems a major mark of distinction, and aside from expressing sudden pauses and jamming phrases together, we know these dashes often add intense emotional tension to the ideas being expressed.

To examine that particular dash at the end of the second line, let's look at it in the context of the whole stanza:

"A Bird came down the Walk—
He did not know I saw—
He bit an Angleworm in halves
And ate the fellow, raw,"

Here, that dash seems to force us to pause right at a critical moment. The speaker is watching the bird, waiting to see what happens--enter the dash, where we hold our breath, too, waiting to see-- and in the next instant, the bird eats the worm.

So, forcing us to pause and consider gives more weight and drama to the action that occurs directly after the dash, namely, the bird devouring the worm. It makes us consider that action a bit more than we might without the dash. That tension calls attention to the viciousness of the bird's actions: he's not just eating the worm but is biting him in half and then eating him raw. And, the dash also calls attention to what came immediately before it, so we're particularly aware that the speaker is watching this feral scene like a spy, unknown to the bird.

In short, the dash adds tension to that moment and accentuates the tenuous relationship between the person observing the action (the speaker) and the one performing the action (the bird).