It completes your gates
You have one gate of a channel, and the transit activates the other. The channel closes — and for as long as the transit lasts, you have that energy as your own. This is the strongest influence: a new ability or a new pressure appears that wasn't there before.
It amplifies your gates
The transit has the whole channel, and you have only one of its gates. The theme of that channel sounds louder in you than usual, but it comes from outside, like a strong wave.
The transit's own channels
The transit forms a channel by itself, without your gates. This energy is “in the air” for everyone: you notice it in the people and events around you, but it doesn't become part of you.
Temporarily defined centers
If a transit closes a channel that leads into one of your open centers, that center becomes defined for a while. For example, an open Solar Plexus may live through an emotional wave as its own for a few days. Knowing this makes it easier not to identify with a temporary state.
A transit that defines your open center isn't a mistake in your chart. It's a hint: this is how that energy feels when it's consistent. Get to know it, but don't build your identity on it.