"I've gotten more mature and I've changed my mechanics but it's all about maturity in the mind. It's all about going after hitters and not backing down," Jeffress said.
Commanding his blazing fastball has been an ongoing challenge for Jeffress, and too many bases on balls led to his problems last year with the Royals and after he was sent to the Minors.
But Jeffress has improved his control, he's also added a slider to his fastball, curve and changeup, and then there's that new outlook.
"Maturing pitching-wise, developing that state of mind of just going after hitters, pitching inside. You pitch inside you might throw a ball as well but just going inside, pounding the zone and keeping the ball down -- that's what I've been doing lately," Jeffress said.
He's doing more thinking and not just out there throwing hard, he said. But have the changes taken anything off his fastball?
"I don't think I've lost any velocity," he said. "It has a tendency to sink and the movement is better, even if it's not as hard."
Jeffress saw his first action since being recalled from Triple-A Omaha in Sunday's 5-3 loss to the Blue Jays and took a hard shot off his pitching arm in the ninth inning. The ball ricocheted to third baseman Mike Moustakas who threw out batter Jose Bautista.
After taking some throws to test his arm, Jeffress stayed in the game, but gave up a double and a walk, and was relieved by Aaron Crow.