Pin your arms, kill the cheat, build the peak.
Don't fully slam into a locked elbow at the bottom, keep a hint of tension to protect the joint.
With your upper arms flat on the pad, the preacher curl deletes every trick your body uses to cheat a curl, no swing, no shoulder drift, just biceps flexing the elbow against the bar. The angled pad loads the muscle hardest in the lower, more stretched part of the rep, exactly where standing curls go easy. It is the strict standard for isolated biceps work with a bar in your hands.
Both work. A straight bar keeps your palms fully turned up for maximum biceps demand, while the EZ bar's angles are kinder on wrists and elbows, so pick whichever lets you train hard without joint complaints.
Until your arms are nearly straight, stopping just short of a hard elbow lockout. That keeps tension on the biceps and spares the joint at its most vulnerable angle.
The pad angle means the load peaks while your biceps are lengthened, the opposite of a standing curl. That stretched-position work is why preachers are so effective, and why the weight has to be humbler.
