Electrokinesis is the power to control and manipulate electricity. With this power, merpeople also can conjure lightning from the sky on any target and start a fire, if so desired, as shown by Rikki when she conjures lightning from the sky accidentally for the second time in the series. Rikki also conjured lightning that hit the ground, only to frighten Charlotte. Merpople also can cause explosions in electronic devices that depend on electricity, like cellphones. It's the amplified version of hydro-thermokinesis that merpeople receive in the Moon Pool during a planetary alignment.
In "Control", Rikki looks at the full moon and swims to Mako Island with the girls and was in the Moon Pool during the special planetary alignment which magnifies her original power to electrokinesis, and she ends up making a lightning storm. This can be made stronger with the help of her fellow mermaids.
With the combined amplified elemental abilities of Emma and Cleo, Rikki can create a full on storm that is extremely powerful and very dangerous. This effect is shown in the second season premiere, "Control". Rikki doesn't seem to use this power much in Season 2 other than a few times and it is seen a little bit during Season 3. She mainly uses this power on the wall of the Moon Pool. She also used this on Ryan's walkie-talkie.
The hand movement for this is the same as before, however, Rikki is shown suddenly opening her palm in the chosen direction when summoning lightning. This may signify the sudden release of energy needed when conjuring lightning. Rita Santos and Nixie are shown using this power in "Battlelines". Rita is shown using this power to create a storm in a jar when teaching the girls. Nixie is shown using this ability when she creates a storm in a jar as well.
In "Stormy Seas", Sirena and Ondina have a contest mermaid powers and after Ondina whipped up a storm over the ocean, Sirena used it against her and conjured lightning.