When the pectoral muscles are visibly flexed. This often looks like a "groove" on the upper section of the chest. 

Can be visible involuntarily when simply moving, especially with [[shredded muscles]] physiques (low body fat percentage) or voluntarily to create a [[bouncing pecs|bouncing effect]]. 

h4. Related tags 

* [[bodybuilder]]
* [[side chest pose]] 
* [[double biceps pose]]
flexing pectorals