Narrow overhand grip for thick forearms and outer biceps.
Keep your wrists rigid and straight, don't let them collapse under the load.
Turning your knuckles to the ceiling hands a big share of the curl to the forearms, while the close inner grips pin your elbows to your sides and keep the bar path strict. The biceps still drive the movement, just from their weakest grip position, which is precisely the point. It fills the gap that palms-up curling leaves: thicker forearms and a grip that keeps up with your arms.
The overhand grip takes the biceps out of their strongest position, so the smaller forearm muscles carry more of the load. Being well down on your normal curl weight is expected, not a problem.
Mostly the path and the discipline. The narrow inner grips make it much harder to flare the elbows and cheat, so each rep stays honest even as the forearms fry.
They are folding backward under a load they cannot yet hold flat. Go lighter until your knuckles stay level with your forearm through the whole rep, the EZ angles help but they cannot rescue an overloaded bar.
