This is the second part in a series that spotlights three 49ers-Seahawks matchups to watch Thursday, 5:20 p.m. (NFL NetworkKGO-TV, Ch. 7), at Candlestick Park.49ers special teams vs. Seahawks RS Leon WashingtonA year ago, the 49ers won the battle for field position, in large part due to their exemplary coverage of punts and kickoffs.Through six games, any time the 49ers have booted the ball away, it's hold-your-breath time.The 49ers rank 31st in covering kickoffs and they are 30th on punt returns. And they face one of their biggest challenges Thursday evening in slowing down veteran returnman Leon Washington.Washington ranks behind only Minnesota's Percy Harvin with a 31.7-yard average on kickoff returns. Washington is second all-time in NFL history with seven kickoff return touchdowns.If veteran kicker David Akers, who tries to drive his kickoff low and through the end zone, leaves Washington some room to return kicks, the 49ers must do a better job of preventing game-changing plays.One of those such plays occurred at the beginning of the second half Sunday against the New York Giants, who led 10-3 at the time. Rookie David Wilson took advantage of a huge hole formed when the Giants' two-man wedge walled off the 49ers' jumbled pursuit team. Wilson returned the kickoff 66 yards to set up a touchdown that set the tone for the remainder of the game."We know that's our job to cover those kicks and get them contained and get them covered," said 49ers coach Jim Harbaugh, who could not provide a reason for the team's rash of lapses on the coverage units.The 49ers made several offseason decisions that appear to have greatly impacted their coverage units.The 49ers did not attempt to re-sign Blake Costanzo, who was second on the team with 17 special-teams tackles. Instead, the 49ers signed veteran Rock Cartwright, who was cut loose during the team's final cuts. Another key contributor, Colin Jones, was traded to the Carolina Panthers for a future draft pick.The 49ers' top tackler on special teams, C.J. Spillman, has yet to record a tackle through six games. The 49ers' top special-teams tackler, Tavares Gooden, has been nursing an elbow injury. He is scheduled to play in Thursday's game.