Thus, the goal of the "extract" stage is to "concentrate" the possibly dispersed entropy of the input keying material into a short, but cryptographically strong, pseudorandom key.
HKDF-Extract(salt, IKM) -> PRK PRK = HMAC-Hash(salt, IKM)
salt may be 0. In this case, it feeds
IKM into HMAC with a fixed key:
PRK = H(opad || H(ipad || IKM))
How to envision this as a concentration of entropy? How is this different than simply hash the IKM:
PRK = H(H(IKM))
PRK = H(IKM)