One difference between tower bell ringing and handbell ringing is that a tower bell has its own momentum and rhythm, but a handbell doesn't. On handbells, all of the rhythm has to be created by the ringers. We have to start at the right speed, absorb the rhythm of that speed, and keep our bells going in that rhythm. The most important point is that we have to anticipate: we have to know which places our bells are going to ring in, know when those places are going to arrive, and start moving our arms early enough to hit the places accurately.
All of this also applies to tower bell ringing, but because handbells don't have the delay from the rotation of the bell, it is possible to ring handbells (badly) by waiting to hear the correct number of other bells ring, then ringing our bell. This more or less works on six, although it certainly doesn't produce fluent rhythmical ringing. It's just about possible to cope with it on eight. On ten or more it's a disaster. It's interesting to calculate the interval between bells, for each number of bells being rung, assuming a reasonable overall speed. This table shows the number of bells, a reasonable peal time, and the corresponding inter-bell interval (in seconds).
The interesting thing is that the intervals are similar to typical human reaction times. There are different sources of information about reaction times (for example, an experiment based on ruler-dropping), but if we take into account the need to swing a handbell, it seems likely that hearing a bell and then ringing a bell to follow it will take at least 0.2 seconds, maybe longer. I think this helps to explain why "reactive" ringing on six is possible, although slow and lumpy, but on ten and twelve it's essential to anticipate and aim for the places before they arrive.
Naturally, being able to anticipate and aim for the expected arrival of the position in which you should ring assumes that the ringing is smooth and rhythmical, so that the positions actually arrive when they should. If everyone is ringing perfectly then it's easier for everyone to continue ringing perfectly; too much disturbance and it all falls apart.