Workers can fade through units (unless they are sieged) when they collecting resources (minerals, vespene). So Idra commanded his drones to mine minerals and canceled the command shortly after.

This is common technique, mostly used to get scouting drone back through a walloff (own unit stands on "hold" in a small choke) or when a mineral line is under siege (e.g. Siege Tank, Liberator), the player can select all his workers to mine a single mineral patch (has to be spammed, otherwise workers redistribute) outside of the siege range until they siege is cleared up.

That's why he apologized when he won.