As noted earlier in the season, Nick Swisher (NYY) was struggling from the left side of the plate before going on a small run. He currently holds a .414 wOBA from the right side compared to .320 from the left, but his HR rate is nearly identical at this point in the season.
One of the big disparities for Swisher is his contact rate. Since 2010, Swisher has had a tougher time making contact against right-handed pitching.
This season, Swisher is making contact on 87.0% of his swings versus righties, compared to just 75.8% versus lefties. This isn't a change from his rates in the two previous seasons (84.0%/74.8%). However, his power from the left side has dropped. He's slugging just .370 from the left side this season, compared to .453 between 2009 and 2010.
In the last month, Swisher has been doing much better from the left side, slugging .525 with 3 HR. So perhaps the first few months of the season were just a blip, rather than a sign of decline.