When taking samples, one has to tap it onto an EVO which causes a small electrical jolt to emit from it, meaning the process is complete. It's a small rod that comes in a containment worn around the waist, similar to a fanny pack.
As mentioned before, it had always been something that Holiday wanted to do. Once it was given to Rex, he had taken many samples along the way. It especially came in great use when he was pinned down by NoFace and almost prevented him from escaping The Bug Jar. Using the small jolt from the scanner, it stunned NoFace, causing him to fall.