I'm not sure if it's what you are referring to but when I do BI's I always barely do 2 sets of these..
2 DB , one in each hand.
Hold one DB, palm up, off of your thigh so the arm is 90 degrees bent. Sort of like where it would be 1/2 way through a standard curl. Keeping Tension in the Bicep
Curl the other DB for a set of 10 while still holding the other DB.
Immediately Switch DB positions and Hold the other one while you curl for a set of 10
You may catch yourself letting the DB rest against your thigh. Hold it up and cry. Especially on the second set.
I feel it for 3 days after pushing myself with these.
This and 21's kill me. Anyone done those?