The rule of thumb when training biceps is to keep your elbows still! Tightening the armpits will make it more effective.
If you want to work the biceps firmly, be careful not to move your elbows. If the elbows are not in a fixed position, the muscles may recoil or the load may escape, making it difficult to work the muscles. Basically, the rule of thumb is to keep the elbows fixed right next to the body, and to prevent them from moving, tighten the sides of the arms. If you can’t help but move, I recommend doing arm curls while holding a towel between your arms as shown in the image below.
