You use a chocolate spray gun, which is filled with a 1:1 ratio of chocolate to cocoa butter (or 60:40 white chocolate to cocoa butter), melted and mixed together, allowed to cool to 35C, and then sprayed onto frozen or chilled products, usually mousse based or other similar desserts.
This particular velvet texture is from frozen desserts being sprayed /img/vbsmilies/smilies/smile.gif
As far as tricks go? There aren't really any, it's a pretty simple technique. There's a dial on most guns that changes the spray to finer/coarser depending on your tastes; I tend to go fairly fine.
Tips? Don't hold the spray gun sideways because it splatters thick chocolate inconsistently onto your product. Work quickly, and rotate your item for even spraying. Most importantly, probably, is that you should always, and I mean always, clean out your gun with hot soapy water, spraying the nozzle until the water runs completely clear; cleaning a nozzle with set chocolate jamming it is a ridiculously tedious job that is easily avoidable.