Potency, and Critical Hits

[Estellios]

Autoattacks seem to keep going while using other skill, not stop. What you are likely seeing is the skill Critting (45), followed by a normal hit of 15 damage (Autoattack). I've noticed this a lot on Archer.

This is correct. As long as you are in auto-attack range you are auto attacking at a constant rate, even in between using skills.

Potency is mostly a means of figuring out the relative power of skills. For the record, 100 potency is the damage your auto-attack does. So if your auto-attack does 50 damage, a 150 potency skill will hit for 75 damage.