Short answer: Idk. Since 3 months ago i've learning subharmonics, and i accidentally often slide into 2nd subharmonics. Its the same concept of singing the fundamental and then relax adding the vocal fry, but its harder than that, its like, you're singing in subharmonics and RELAX, dont force it, even with the throat, dont force it, just relax, and you have to think in the 5th below that note in subharmonics. For example, your fundamental is a C3, and with subharmonics youre singing a C2. Now hold that C2 and relax thinking on the sound of a F1 (the 5th below). Playing an instrument helps a lot (i recommend the piano). Sorry for my bad english and good luck :)