In this case, we say "hold on to" or "hold onto" not "hold on". You're right about the common usage of "to" as an indication of motion. I don't know why "to" is used in this phrasal verb but there may be a sense of movement towards the thing you hold. "Hold on" is used without "to" as a synonym for "wait" (normally in the imperative form) when you are asking someone to wait, perhaps when on the phone. e.g. Hold on - I will find the information you need. Enjoy this song from ELO "Hold on tight to your dreams." https://www.youtube.com/watch?v=Gt_Se7BtSQg else

To 'hold onto' means to hold something firmly: 'On a windy day, hold onto your hat'; 'In a crowd, hold onto the children'. You can hold onto abstract things too. 'When life became difficult, he held onto his dreams.' http://images.wookmark.com/196248_the-best-thing-to-hold-onto_large.jpg

Yes, this is somewhat tricky. 'Onto' is defined as "toward a surface that is on" http://www.learnersdictionary.com/definition/onto You explained it yourself. "The witch holds on the cane" sounds somewhat awkward to the ear. Even if I change to the past tense, it sounds awkward "The witch held on the cane". If I say: " The witch holds the cane" or "The witch held the cane" these mean that the cane is/was in her hand as if she is/ was carrying the cane but not holding onto it while she was flying. I know it is not clear, but I hope it helps. Jamie

“Hold on" is a phrasal verb with several different meanings, such as continue. "Hold onto" is the verb "hold" plus a preposition, and the preposition needs an object, like a cane. Thus, there are several possibilities, some of which are practically the same. (This is why phrasal verbs are difficult.) One could say "hold onto the cane" or "hold on to the cane". Same thing. I guess you could also say "hold on the cane", but that does not sound natural to me, though grammatically correct.

Although there has been many answers, I'd like to mention your understanding of "to" seems not complete. You say: Because 'to' kinda gives it a feeling of 'direction'. Like, 'toward' the cane. In addition to the sense of "direction", "to" also implies that the destination is "reached". For example "They came *to* New York" means they reached it, or they "arrived in it". With that in mind, both "hold onto the cane" or "hold on to the cane" make perfect sense. You hold "firmly" (on), physically reaching/connecting *to* the cane. "Hold on" alone doesn't include this explicit "connection", or "touch" to the object.