For me, the follower has to have the "right" kind of embrace (whatever that means, although some type of head contact is helpful), plus she has to do the "surrender" thing. My part (at least partially) is to simplify what I do, and not disrupt things. The steps really become unimportant (mostly walking & rock steps, and maybe an occasional turn or ocho).
If I don't feel that kind of embrace, then after a feeling out period, I'll be dancing more for the steps (rather than the connection). i.e. Going for fun, rather than something deeper.
That pretty much sums up my experience at the moment but I would say the connection as well as the embrace.
Over the past two weeks I been away on two tango festivals (12 days and 5 days). There were three partners I felt like that with. They had different body shapes, different embraces and different styles but the connection we had allowed our movements to be totally smooth, with no feeling of the slightest pull, jerk or anything like that. Three totally smooth partners but three totally different styles.
The other thing that surprised me was that they were not dancing as frequently as others so I can only assume others did not get the same connection I did. Personally I would would have been happy dancing with any one of them all night.