It's springy so you squash it - pop it in and it springs out - your natural muscles hold it in just like they hold a tampon.
The difference betweent he soft cups and the mooncup is that you can have intercourse when using softcups.
Soft cup also do not have a "stem" which the mooncup (divacup/keeper) does.
The stem allows you to pull the cup out (sometimes you get a little suction and you need to break that to allow removal).
Because of the stem you are less likely to get spillage.
The smaller capacity of the mooncup allows for you to only have to empty it a couple times a day, so the larger capacity of the soft cups would (in my view) encourage people to leave them in longer - hence the greater chance of infection.
However, if you really do miss having sex when on your period and don't want messy sex - then the soft cups are a great idea.
They keep everything clean and form a barrier against the flow.
They are not however contraceptive and should not be used as a barier method of contraception.
Everything has it's good and bad points to be honest.
While I really want to try a mooncup, the instead soft cups just don't do it for me.