Answer by taking a quote from the following article:

Yelberton Abraham Tittle Jr. (October 24, 1926 - October 8, 2017), better known as Y. A. Tittle, was a professional American football quarterback. He played in the National Football League (NFL) for the San Francisco 49ers, New York Giants, and Baltimore Colts, after spending two seasons with the Colts in the All-America Football Conference (AAFC). Known for his competitiveness, leadership, and striking profile, Tittle was the centerpiece of several prolific offenses throughout his seventeen-year professional career from 1948 to 1964. Tittle played college football for Louisiana State University, where he was a two-time All-Southeastern Conference (SEC) quarterback for the LSU Tigers football team.

Tittle threw the ball from a sidearm, almost underhand position, something novel at those times, though it was common practice in earlier decades. It was this seemingly underhand style that drew the curiosity and admiration of many fans. In tandem with his baldness--for which he was frequently referred to as the "Bald Eagle"--he made for a very striking personality. Despite his throwing motion, he had a very strong and accurate arm with a quick release. It was because of his quick release and ability to read defenses that he became one of the best screen passers in the NFL. He was a perfectionist and highly competitive, and he expected the same of his teammates. He possessed rare leadership and game-planning skills, and played with great enthusiasm even in his later years. "Tittle has the attitude of a high school kid, with the brain of a computer," said Giants teammate Frank Gifford. Baltimore Colts halfback Lenny Moore, when asked in 1963 to compare Tittle and Colts quarterback Johnny Unitas, said:  I played with Tittle in the Pro Bowl two years ago, and I discovered he's quite a guy ... He and John, however, are entirely different types ... Tittle is a sort of 'con man' with his players ... he comes into a huddle and 'suggests' that maybe this or that will work on account of something he saw happen on a previous play ... The way he puts it, you're convinced it's a good idea and maybe it will work. John, now, he's a take-charge guy ... He tells you what the other guy's going to do, what he's going to do, and what he wants you to do.  Tittle's most productive years came when he was well beyond his athletic prime. On his ability to improve with age, he credited a feel for the game that came from his years of experience in the league. "If you could learn it by studying movies, a good smart college quarterback could learn all you've got to learn in three weeks and then come in and be as good as the old heads," he told Sports Illustrated in 1963. "But they can't."

what was his biggest accomplishment?
It was because of his quick release and ability to read defenses that he became one of the best screen passers in the NFL.