He seems to get a little swing, but then seam movement away from the right handers. The small bit of swing means that the batsmen start to follow the ball with their hands, and then he takes the edge with the seam movement. He doesn't swing it enough that they play and leave, but rather just enough to draw them into the shot. (Most balls seem to only swing about 5 cm - it's the movement off the seam that makes him dangerous). However against the left handers this small swing means that if it's straight, they tend to defend it, and if it seams back, they just hit an inside edge into their pads.
So I thought I'd go through his (brief) career so far and see if my feeling is correct. Here are Shaminda Eranga's test statistics against right and left handed batsmen (prior to the Melbourne test):
|Hand||Average||Strike Rate||Economy Rate|
There is a significant difference.
Of players who have bowled in at least 20 matches, the best strike rate ever is Sid Barnes, with 41.6. Next is Dale Steyn with 42.0 and the Waqar Younis with 43.4. Eranga to right handers has a better strike rate than any of these.
However the other end of the tail we see a bunch of part timers. With a strike rate in the region of 195 are the likes of Shivnarine Chanerpaul (193.3) and Geoff Howarth (204.6). Even Geoffrey Boycott had a bowling strike rate of 134.8. These are good people to be compared to if you are talking about defensive technique when batting, but not so much if you're talking about bowling.
If Eranga wants to make the most of his obvious talent, he will need to figure out how to bowl to lefties. He is a fantastic prospect for Sri Lanka, but they can't keep picking him if he's going to be two bowlers: Steyn to the right-handers but Boycott to the left handers.