In his lifetime, Lorca was acclaimed for his public readings and lectures. They tended toward one of two subjects: his own poetic inspirations or histories of certain Spanish artistic traditions. Naturally, those two subjects frequently overlapped.
One of the most telling subjects he lectured on in his later years was the duende, a mythological Spanish demon that came to signify "spirit" or "soul" in the Spanish arts. It is useful to understand Lorca's interpretation and thoughts on the duende, since they provide insight into both his Spanish roots and his artistic sensibility.
What follows is based largely on extant copies of Lorca's lecture, which can be accessed in full here: http://www.poetryintranslation.com/PITBR/Spanish/LorcaDuende.htm: The Duende
Lorca defines the duende as one of the truly magnificent inspirations for great art. He contrasts it with the "angel" or the "muse," which he says fly above man's head encouraging us to reach. The duende comes down to us when it smells death and brings with it an inimitable authenticity and spirit.
In his lecture, he tells a story of how the duende works. He tells of a time when a polished, talented flamenco singer produced little audience response, but when the next singer, who lacked the polish but had a severe and intense style, came up, she won great acclaim. Lorca attributes this success to the duende, which he believes gave a dark authenticity to the second singer. Talent and style – the "form" of art – do not produce greatness but instead, "the marrow of form" is what produces greatness, and this can only be brought by the duende. He explains that the second singer "banished her muse" and thereby was able to be filled with the duende.
His sense of the duende helps us understand The House of Bernarda Alba in many ways. His idea that great art seeks to create "the dark sounds" sthrough the duende provides insight into the play's dark themes. He describes it not as a style, per se, but as an instinct that is best suited for the arts that use human bodies. The reason for this is that the duende comes when it senses death, which could also help to explain Lorca's interest in stage silence and profound stillness in his theater.
Though Lorca claims that any art in any country can make use of the duende, he believes Spain has special access to it because of the country's close connection with death. This is reflected partly in its love of bullfights. In the duende is contained the "buried spirit of saddened Spain," wherein "a dead man in Spain is more alive when dead than anywhere else on Earth." Again, this sense of death as connected to his homeland provides an interesting way to understand The House of Bernarda Alba.
If this discussion seems imprecise, that is due to Lorca's refusal to nail down a strict definition of his subject. The duende is a spirit to be glimpsed, not a concept to be understood. Consider his definition, which closes his lecture: "The duende… Where is the duende? Through the empty archway a wind of the spirit enters, blowing insistently over the heads of the dead, in search of new landscapes and unknown accents: a wind with the odor of a child’s saliva, crushed grass, and Medusa’s veil, announcing the endless baptism of freshly created things."