Far as I know, flails as depicted weren’t really used in war. They may have showed up in tournament fighting, but not war. The flails that did get used in real battlefields were pole weapons. Long shaft like five feet or so, very short chain that’s essentially just a hinge, then a sort of long head something like 18" that may have had spikes or bumps.

Talhoffer wrote a manual for them, calling it a peasant flail. It has a lot of wonky binds that I would conjecture probably didn’t get a huge amount of use outside duels. In war, it was probably mostly overhead bonk attacks. Fighting against these things in a duel is a real bitch though. A skilled user can shower you in weird bullshit that’s hard to predict or handle and can’t really be done with anything else. There’s a ready stance where you hold it head-down and lean on it.