Both players start by playing two moves each, like normal. In secret, each player chooses one of their starting stones to be susceptible to charming. A group that contains a susceptible stone will switch colour as soon as it touches a group of the opponent that contains at least two stones more.
For example, if the marked stone is susceptible:
and white plays C3:
then the marked group has two stones, and touches an opponent group with three stones, thus it does not become enthralled.
Now, let’s say black plays B2:
When white plays A3, the white group has 4 stones, and the susceptible group only 2, thus it becomes enthralled (which wins white the game, I presume):
Since it’s not known to your opponent which of the first stones is the susceptible one, there’s some psychology involved as well, where it makes sense to bluff that one group is susceptible, while it turns out it isn’t.
Since the opponent needs at least two more stones to enthral a group, it’s not impossible to keep up with your opponent in number of stones, up to a certain point.
Playing in a corner with a susceptible group can be very risky as well, since it’s hard to add stones to a group that’s enclosed in a corner. Therefore I’d expect some interesting opening moves as well.
Note that there’s no rule against enthralled groups becoming enthralled again to convert back to the original owner. I can imagine some game with two very large enthralled groups trying to get the upper hand over each other.