Yaru ki [やる気] (click to listen) means "Determination" or "Motivation."
For example, "Yaru ki arimasu ka?" would mean "Do you have the motivation/determination to do something?"
The following is one of my fave commercials at the moment by School IE - a private tuition school who helps kids get motivated. They say that they will help find the switch which engages the "Yaru ki" in students.
In the CM below, the boy has no motivation but knows he can do it and searches for the switch on his body - does not find it. He then goes to School IE who discovers it for him. Very well done CM - love it!
My Yaru ki switch has been engaged for many years now. Have you discovered your motivation/determination switch?
UPDATE - this is what happens after the boy runs out without his shirt - two different scenarios below. Thanks to grimbyslayer for the heads up.