snap

The TelegraphにあったAudi A3サルーン（セダン）のreviewからの引用です。「ポキッと折れる」という意味のsnapには犬などが「（…に）パクッとかみつく」という意味があります（『プログレッシブ英和中辞典』小学館）。そこからsnap at someone's heelsは「（人）を追い立てる」（英辞郎on the WEB）という意味で用いられます。ここでは近年のAudiの販売台数の伸びを「狂犬病に感染した犬に追いかけられるがごとく」という比喩で表しています。（Koyamamoto）

Audi A3 Saloon review
Audi has launched a saloon version of its A3 premium compact car, but is it worth the price premium?
In 2006 Audi shut the cats indoors and set off its “product firework”, introducing its Q7 SUV as the first of an explosion of new models that, seven years later and with the launch of the new A3 Saloon, consists of a Heinz-chasing 47 varieties.
Overkill? Not if you look at the sales figures, which continue to climb as if they had a rabid dog snapping at their heels. In 2011 Audi sold 113,797 cars in the UK; last year it was 123,622, and 2013 looks set to top that. Launching new cars and endless further derivatives of those new cars clearly works, which is how we arrive at the A3 Saloon, the fourth A3 bodystyle alongside the three-door hatch, five-door Sportback and Cabriolet (which goes on sale next year).
http://www.telegraph.co.uk/motoring/car-manufacturers/audi/10320388/Audi-A3-Saloon-review.html