SAN FRANCISCO -- It's not that difficult to understand why Santiago Casilla is such an intimidating pitcher.

"You can look at the radar gun," Giants catcher Buster Posey said. "I think that tells you what makes him tough."

According to Fangraphs, Casilla's fastball velocity this season averaged 96.6 miles per hour -- the second highest of all Major League relievers behind only Boston's Daniel Bard (97.9 mph). And sure, a pitcher who has stuff that overpowering should be dominating.

"But," Posey continued, "I haven't ever caught a guy who throws as hard as he does who has [that] type of movement. I'd say that's probably the biggest difference between him and a guy that throws 98 [mph]. But if you throw 98 [mph], you're going to be good, no matter what."

"Good" is an understatement for what Casilla has meant to the Giants this year. While closer Brian Wilson and his black beard have gotten most of the attention this October, and others such as left-handed specialist Javier Lopez have thrived in their roles, the 30-year-old Casilla has had his best season to date, going 7-2 with a 1.95 ERA -- almost two runs lower than his previous career high in 2008.

The biggest difference in this season compared to years past for Casilla has been the curveball he added to his arsenal this winter. Prior to 2010, Casilla spent 10 years in the Oakland organization, posting a 4.76 ERA in three full big league seasons. With the A's, Casilla relied heavily on his fastball, while also occasionally throwing a slider. The problem Casilla ran into -- as do many power pitchers -- was that his slider, ranging from 92 to 93 mph, wasn't a big enough drop in velocity from his fastball to consistently fool hitters.

This past offseason while playing winter ball in the Dominican Republic, Casilla began experimenting with a curveball.

"I threw it a couple times and was like, 'Whoa, this is a good pitch,'" Casilla said. "I came here to Spring Training and wasn't sure if it would work because of a different ball, but it worked and I was like, 'OK, I've got it.'"

Casilla's curveball revelation has spelled trouble for opposing batters, who have hit only .208 off him this season.

"It's big. It changes planes. The heater's staying on one plane and the curveball is just a different look," Posey said of the importance of the curve. "When you throw as good [of] a curveball as he has and you have to respect the 98 [mph], it's going to be tough."

A versatile reliever who can eat up middle innings to bridge the gap to Wilson or come in during pressure situations, Casilla also has avoided many big innings. In his 52 regular-season appearances, he had only four in which he allowed more than one run. Since giving up three runs to Arizona in two-thirds of an inning on July 23, Casilla has allowed only four runs in 36 1/3 innings -- including the postseason -- for a 0.99 ERA.

His postseason debut in Game 4 of the National League Division Series -- where he allowed only one hit and struck out two in 1 2/3 innings -- only highlighted what Casilla has done all season.

"He throws lightning bolts. And then when you come with the breaking stuff ... and it's nasty, too," fellow reliever Sergio Romo said. "It's not like it's just, 'OK, give me a break.' It's a plus pitch, so you sit there and it just adds that much pow to his bang. He's pretty unbelievable."