It doesn't have to be done with earphones. Stereo helps. Your brain sorts it out. It also allows the listener to use the balance to get one language only should they want. The idea is to let you use material that might be a bit over your head if I were monolingual. Also, it takes advantage of a simple math equation: It's better to learn 10% of 1000 words than 100% of 50. The listener also gets the benefit of context, which many find helpful in vocabulary acquisition.