For the first part, as Garthor hinted, look up image objects (images) and then the images var in the DM Reference, and if further info is needed, search for info about using them using the forum search.
To do what you want, you'll want to give each mob (person) an image object attached to him which is created after the mob is created (using the New() proc). You'll also want to store all of these images in a global list so you can access them easily using it when you need to either show all these images to someone (because he activated the technique) or unshow them. Both can be done in one statement using client.images and list functions, the showing can also be done with the << operator and an image reference or a list containing those.

The 2nd part has been covered a lot before, like most questions, so use the forum search to look for threads asking similar questions using keywords like "control a mob, "controlling", etc. The gist of it is: to override player movement you use the client/Move() proc. You also want to make something happen when the user of the technique moves, so you'll want to also override his atom/movable/Move() proc to do so as well (you could also use client/Move() for that as well, that would be a little easier but generally it's somewhat less correct for this purpose).