They still have to explain why you have been detained, right?

From my experience having been arrested and held like this before: No. They don't even have to attest to probable cause until the 24-hr limit hits, in which case they will then formally arrest you by taking you to be fingerprinted, a reading of Miranda, etc along with paperwork containing the attestation of probable cause, or let you go, again, with a pile of paperwork, and sometimes both, in which case they arrest you formally, give you paperwork, and an issued summons to appear in court before sending you on your way.

No