Hi,
1. It depends on how or where this is setup exactly. If you are referencing the results page or an archive page here, then I suggest trying the results page live loading feature.
2. I’m afraid it is not documented, but you can get them very easily via the developer tools on your browser. Just in case, here is a screenshot of how exactly it is constructed. The actual checkbox is not visible (because it is not possible to style), but there is an overlaying element, which is displayed based on the checkbox state:
.asp_option_checkbox:after {
opacity: 0;
}and
.asp_option_inner input[type=checkbox]:checked~div:after {
opacity: 1;
}