It's the capacitors charging, totally normal. Some Chinese ESCs are starting to have a capacitor pre-charge cord attached to them which has a spark arrestor like Nautiboyz linked to to charge the capacitors a little slower before you plug in the main leads. The more voltage and the higher Farad rating of the capacitor, the bigger the spark.
Put a Etti Cap board on that bad boy. Then you'll really see a spark.
Seriously, there is some debate on whether or not a spark arrestor is beneficial. I havent heard of anyone distroying a ESC with or without them. So the way I see it, whatever floats you boat.
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