It's true that if you are holding the violin correctly, you should be able to support it comfortably between your collar bone and your chin without putting your hand on it. However you must not "squeeze" the instrument in any way, either by pushing down with your head, or hunching up your shoulder. Your shoulder and neck must be relaxed or tension will migrate to your arms, wrists and fingers, preventing them from working optimally.
With your shoulders relaxed and your arms hanging down naturally by your sides, it should be possible for the instrument to rest on your collar bone, and to be held in place simply by the weight of your head. For most people, the gap between the bottom of the chin and the collarbone will be greater than the depth of the violin body, so the rest of that gap must be filled using some combination of chinrest and possibly shoulder rest.
The appropriate size and positioning of both chin and shoulder rests depend on your own personal anatomy, particularly of course the length of your neck.
There's some very good advice at this link (there are also people, with whom I have no affiliation, trying to sell you things): http://www.artistinbalance.org/vib/