When you engage a lever, it shortens the string so that the string plays 1/2 step higher. It enables the player to play in different keys without having to retune the harp for each key. I think you should read as much as possible about lever harps and levers. There are sites on line which discuss different types of levers and what they are used for. Do a google search. There are different tunings for lever harps so that you can get optimum usage with various configurations of the levers.
Personally, I would look at other harps besides the “sicle” harps. They are okay if you have no strength in your hands, but the tension is very very floppy. As I understand them, they come unlevered (harpsicle), levered with selected levers for flat keys (flatsicle) and levered with selected levers for sharp keys (sharpsicle) and then there is one with full levers (fullsicle). They all have the same loose tension from what I have read recently.
Briggs