In the Chanukah 5777/Winter 2017 issue of Kosher Spirit, the answer that was given to the Question for the ~ needs further clarification.
Lettuce can be soaked, washed and checked for insects on Shabbos and Yom Tov with diluted soap. This is because there isn’t a certainty that there are any insects on the lettuce. However, this should be done immediately prior to the meal.
If one does find an insect on a leaf, they should either discard the leaf, or shake the leaf until the insect falls off. One should not wash the insect off the leaf.
Please note that this is applies to lettuce, which is generally clean, and the sole purpose of soaking and washing the lettuce is for insect checking. If the lettuce is full of dirt then it cannot be soaked in order to remove the dirt, as that is an act of Borer. One may wash off the dirt under running water immediately prior to the meal.