The e.l.f. Buffing Foundation Brush is put to the test by Marcela in her video review. Following the instructions, she pumps foundation onto her hand and applies it onto her skin using a circular buffing motion. Marcela is impressed with the brush's ability to create a flawless and airbrushed appearance. She loves how her skin looks after using the brush, describing it as flawless. Overall, Marcela is highly satisfied with the e.l.f. Buffing Foundation Brush and recommends it for achieving a flawless makeup look.