In part modeling, you can use "1 degree of freedom under-defined sketches" to easily move bodies relative to each other using "Instant3d".
Model the fixed body. In the example of the two cylinders, model the fixed cylinder by "extruding a circle" or "revolving a rectangle".
The moving body should be modeled in such a way that the main sketch can be moved in the desired direction. Define everything in the sketch but don't fully define it. Leave one degree of freedom so that the sketch can be moved in the desired direction. In the example of the two cylinders, the moving cylinder should not be modeled by extruding a circle because a circle with a fixed center cannot be moved. Rather, use a rectangle and define its length and width and make sure it can be moved in the same direction as the moving cylinder. Then revolve the rectangle to make the moving cylinder.
Make sure "Instant3d" in on. Locate the green arrow on the moving cylinder and move it! This motion will not change any geometry because the moving sketch is "defined enough".