Dante is describing sinners suffering in boiling pitch. Their suffering is eternal; they suffer in the pitch, and they suffer when they break the surface.
There is a pool of boiling pitch that the souls bob in and out of. When they break the surface, a demon grabs them and tears them apart. By remaining under the pitch for as long as possible, they can escape the demon's tortures and gain a form of peace. (This is a relative form of "peace" because they are still submerged in boiling pitch.)
When a demon does pull a sinner out, Dante describes him being pulled out by his hair. The demon, with the sinner hanging out of his mouth, appears to look like an otter.
