When used rectally, the KM-503 vaginal/rectal probe should be placed at least deep enough so that the metal rings are fully inserted and touching skin. This would be the most shallow insertion. You would insert in this location if you cannot insert deeper due to anatomy or if you are more interested in working the perineal muscles which are closer to the surface. The probe can be placed all the way in so that the anus closes around the narrow tail end of the probe with the wires still fully exposed. This is the deepest insertion. Do not insert the wires into your body. If they are drawn in during work simply power off your toner and pull the probe gently back into position before continuing. The deeper the probe is inserted, the more comfortable the stimulation may become. This isn't the case for some users, so make sure that you follow your own comfort.

The smaller KM-5018 rectal probe positions the electrode panels very close to the anal opening. This can help activate muscles that are closer to the perineum as well as the pelvic floor. The "vibration" sensation you are describing is the rapid contraction and relaxation of the muscles as there are no moving parts in the probe. You can often feel the current transfer stimulation down nerve branches to activate muscles in more distal areas as well. All normal and expected. Over time this stimulation will grow more comfortable and you will be able to increase the mA's higher with practice.

You can always choose to manually squeeze as your toning delivers current, but this isn't necessary. You will likely be activating more of the perineal muscles rather than the pelvic floor, but this has benefits as well. Exercise in any form is healthy for most people. Note that the pelvic floor is a deeper muscle and more difficult to isolate. Think of how it feels to stop the stream of urine while voiding. This is a very small internal contraction. THAT is your pelvic floor and the muscle that the electrode is stimulating.