Theacrine is a small alkaloid molecule which can be seen as a structurally modified version of caffeine, since it appears to be synthesized from caffeine in some plants and then accumulates; it is found in highest (known) levels in camellia assamica variant kucha which is from where Kucha tea is made from, but we are not certain if this is the largest or only source due to a lack of breadth in the research.
The mechanisms of theacrine parallel that of caffeine for the most part, and while it seems to have a stimulatory effect in research rodents it occurs at a higher dose (and the exact oral dose where it peaks with theacrine is not known). Similar to caffeine there is a sedative effect at relatively low doses, but where this sedative effect with caffeine is at an impractically low dose with theacrine it is the dose normally consumed by tea; this may underlie why Kucha tea tends to be recommended for relaxation more than stimulation.
The one study that noted stimulation with theacrine failed to find any tolerance over the course of seven days, a time frame where caffeine would normally show tolerance. This suggests that the body either does not or may have a reduced tolerance to theacrine, but this requires more research (including oral studies) to assess further.
At this moment in time, theacrine looks interesting due to being one of a few compounds to be involved in adenosine signalling but not enough research exists to pinpoint where it may or may not be useful as a supplement.