Several possible reasons for his behavior:
1) He's tired of getting spanked like a schoolgirl at the track by stock 5th Gens...
2) He just stole the car and there you are being a d-bag drawing attention to him while he tries to flee...
3) He's a proctologist and he's really cheap with advertizing dollars...
4) He just left the proctologist and he was in shock and trying to convey his pain...
5) Maybe he was being nice and waving back but all the other fingers on that hand we're shot off in the war...
6) He might have been from a country where that finger means...ah scratch that it still means what it does here
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418 LS3, custom cam, tuned vararam, ported tb, Kooks stepped LT's, Kooks Hi-Flo cats and 2.5" street sleeper cat back, 18% UDP, crap stock rear tire size 12.03 @ 117 mph.