The incident was caught on camera and the video was recently leaked. The Chinese fishing boat clearly turns and guns ahead to ram the Japanese boat at 2:15.
There is a story in The Economist:
As the actual video shows plainly (see minute 2:15), the captain gunned his engines and rammed his boat into a Japanese patrol vessel that was trying to stop him from fishing in water Japan considers its own, near the Senkaku Islands (the Diaoyu, in Chinese). The islands themselves can be seen in the distance. As symbolic evidence of growing Chinese assertiveness in its neighbourhood, the footage can hardly be beaten.
The government should have released the video publicly, perhaps in a gesture of defiance, just as it was freeing the Chinese captain. Instead it timidly sought to suppress it—and has been left to cringe ever since it was leaked onto the internet. The TV news is now full of policemen huffing and puffing to find the culprit who uploaded the video.
[...]I think this video should have been made public. I don't see why they would want to keep this hidden when it makes them look like they were in the right to seize this boat.