Backerkit seems to be a common name I see popping up around many different circles, and one I intend on looking more into. Obviously they need to get something out of it, so I'm guessing it's a percentage of whatever is being asked for. I wonder if what they offer is dependent on that upfront cost or percentage, but that's more of a question for Backerkit.
So they pay for your advertising, and you pay them back plus their fee when your Kickstarter funds arrive.
They track pledges generated by the ads they run for you, and ask for a percentage on those pledges only. Not of the whole Kickstarter fund, just the ones generated by their ads. I can't remember the exact percentage off the top of my head, but it's not onerous.
The way they manage it is they constantly track your ROAS (return on ad spend) and keep it above a level you specify. For example, you might decide that your ROAS needs to be 2 or higher--that means for every dollar you spend on ads, you get $2 in Kickstarter pledges. They track that and report it to you via a dashboard. So you can't
lose money; at worst, the ads just cost the same as they raised, making it a wash. But they are generally able to maintain a ROAS of 2 or so. If the ROAS drops below your specified level, they stop the ads that aren't cutting it, so they make sure to keep you safe.
Once your Kickstarter ends, you can see clearly how much you spent on ads and how much was pledged from them. BK send you an invoice, and you pay it after two weeks, when your Kickstarter funds arrive.
Like I said, it's not a magic want. We tend to get a 10% boost or so in funding. But that's 10% we wouldn't have otherwise.