Iansan's buff is based on 3 Factors:
Iansan can hold a max of 54 Nightsoul Point, and her buff decreases in value when she's below 42 Nightsoul Point. Consistent movement is needed to maintain good buff uptime.
Iansan's [C4] allows her to frontload Nightsoul Point if it had ever overflowed previously.
Iansan buffs her teammates through her Burst, as well as provide some additional healing for the active character through one of her passives. Characters with movement-based abilities will greatly contribute to Iansan's buff as Iansan's own movement cannot contribute to it. Movement is needed to help Iansan maintain a high number of Nightsoul Point, and the active character gaining and consuming Nightsoul Point can also contribute.
Movement is defined as anything that moves the active character for any amount of distance, whether vertical or horizontal. This includes things such as attacks, Skill and Burst animations, jumping, etc. Almost any kind of movement will contribute, as long as it's not Iansan's own. This also extends to the gain and consumption of Nightsoul Point.
Iansan's [C1] gives her 15 Energy upon consuming 6 Nightsoul Point as long as she's in combat and in Nightsoul Blessing state, effectively changing her Burst cost from 70 to 55. This has a cooldown of 18 seconds.
It is important to note that at [C0], using non-Energy Recharge weapons in teams where Iansan is the only Electro may be unrealistic due to how high her Energy Recharge needs can become. In addition, in teams where movement is scarce, ATK weapons gain greater value.