A B Augmented (Baug) chord contains the notes B, G, D#. It is built from the augmented formula 1 - 3 - ♯5, with B as the root.
Why does B augmented work so well before E minor?
Because it already contains G, the third of E minor. A plain B major has F♯, which has to move a full tone to reach G. Raising it to G in advance means the resolution only has to move the other voices, so the change lands softly even though the chord itself sounds tense.