Swirling a teapot means to move it in a circular motion.
In Japan, this is generally not advised.
The reason is that your tea may become too bitter.
But I felt like experimenting with this, in order to confirm if it always results in a bad outcome.
My findings so far
When you swirl (or shake, stir, etc) your teapot you are essentially speeding the infusion rate.
For example, note how much faster sugar dissolves in water when you stir quickly as opposed to just a little bit.
I used a fukamushi sencha for this experiment because it has a lower infusion time and it’s easy to note the change of the liquor’s color.
The standard way to prepare this tea is one teaspoon of tea for 60 ml (2 oz) of water at 80°C (176°F), with an infusion time of 40 seconds.
The first thing I did was to to start swirling my kyusu from the beginning of the infusion until the end.
At the end of that infusion, the tea had a dark green color and it did taste stronger and more bitter than usual.
So I guess that I swirled for too long.
Next, I tried infusing for 20 seconds in total while swirling.
This time, the taste wasn’t that strong but it was more intense than if I hadn’t swirled the teapot.
Which brings up an interesting point: you can try swirling for a short time in case that you are in a hurry and need to steep a tea quickly.
I think that this may work well for teas with long steeping times that do not go bitter easily.
For example it could work with Japanese black teas (wakoucha), which can be infused for 5 minutes
Another application might be to shorten the time of cold brewing (meaning to infuse in cold water).
But let’s return to my simple experiment.
The final thing that I did was to infuse with the normal time, and just swirl for the last 10 seconds or so.
This time the liquor had an intense color, which I find desirable for a deep steamed tea.
Plus, it did not become bitter.
Sometimes, I personally find that the first infusion of some deep steamed teas comes out too pale.
Shortcomings of swirling
First of all, note that not all teas are the same. Some might get more or less bitter with the same brewing parameters.
The first problem that I find is that it’s hard to be consistent this way, because it adds another variable.
Your results might change depending on how fast or slow you are swirling the teapot.
It’s also difficult to hold the teapot well while swirling if you’re using boiling water, because the teapot becomes very hot.
Finally, if you swirl too quickly you risk spilling some of the tea.