Thicker bar, bigger forearm and bicep demand.
Don't swing. The fat bar already taxes your grip, so keep the reps strict.
Wrapping fat grips around the bar spreads your fingers so wide that hooking on passively stops being an option, turning every curl into a continuous crush-grip effort for the forearms. From the stretch at the bottom to the squeeze at the top, your hands and forearm flexors never get a passive moment. It earns its slot for anyone whose grip gives out before their back does on deadlifts, rows, and carries.
They thicken the bar so your fingers cannot wrap fully around it, forcing the forearm muscles to squeeze constantly instead of letting your hand hook on like a passive strap. The curl becomes a grip exercise that happens to move a bar.
Yes, and it should. The thick handle deliberately makes your grip the limiting factor, so a lighter bar with a crushing squeeze is doing exactly what you bought the grips for.
Roughly, yes. Wrapping a towel around the bar thickens it enough to mimic the effect, it is just less consistent rep to rep than a proper set of grips.
