Sceletium tortuosum is a herb known as Kanna which is traditionally known as a psychoactive herb. It is not known to be hallucinogenic nor habit forming but is taken prior to stressing events such as hunting (traditional use) for its cognitive effects.
Limited evidence does not suggest any improvement in reaction time or many parameters of cognitive performance such as memory although there may be an attenuation of state anxiety. State anxiety refers to the increase in anxiety experienced during a high stress event (such as a cognitive or physical test) and administration of Kanna prior to these events may reduce how much anxiety is experienced and thus the deleterious effects of anxiety on performance.
While the mechanisms as to why Kanna exerts these effects is not conclusively known, it is known to influence the amygdala of the brain (a brain region central in emotional processing) and is known to also have inhibitory effects on both the serotonin transporter as well as an enzyme known as phosphodiesterase 4 (PDE4); both of these proteins existing in the amygdala.
While there is not enough evidence to currently recommend Kanna, its main role of reducing state anxiety appears promising and there are a few other claims (such as antidepressive effects) which remain untested.