Link velocity refers to how quickly you build links to your website. The key to link velocity is consistency. This means that you shouldn’t build or receive just a few links for a few months and then all of a sudden ratchet it up to hundreds or thousands of new links. This will look unnatural and can suggest less than savory practices (see what is black hat SEO).
A natural looking link velocity is an upwards curve because the assumption is that, at the beginning your site is completely new and likely won’t get a lot of links. Over time, your link velocity will pick up as your site continues to build a reputation and grow its audience.
As more people are coming to your site, you’ll see the number of incoming links continue to build, as well, because those people are actively linking to your site and sharing your content with more and more people who will in turn share your site with THEIR audiences, and so on. Over time your site will snow ball because you’ll get more links which will boost your search engine ranking which will in turn bring more traffic and then more links, so it’s a very cyclical process.
Note that that’s how Google believes it should look because it’s natural. If you want to build more links to your site, amp it up slowly. Don’t just begin building hundreds of links overnight but build up to it and then be consistent with it because an overnight drop off will look just as suspicious.