I've done a few, but a lot less than most of my colleagues! You just don't get paid much, or anything in some cases. I spent two weeks on an indie show once and got $52 total. I had to spend more than that paying for parking near the venue. Most profit-share shows almost never reach budget and part of it isn't because they don't have capital (or because they've fudged the books like u/LooksAtClouds has suggested, although I've seen that happen, too!) It's because they know revenue won't extend to paying for the bare basics; venue, rights, production costs etc.
Some profit-share companies are now taking an 'open-book' approach where the whole company has access to a show's financial information, including budget, actual costs, and expected revenue, and can get a better understanding of where things sit. Rafe Beckley has written a useful guide to that approach, which is more honest but doesn't really increase the chance of you getting paid.
If you do get asked to work on a profit-share show, I suggest you check: how many performances they are doing, how many seats the venue has, and how many people are involved. I usually add all this up: (performances * 30-60% of total available seats * average ticket price)/number of people working on show and see how bad this number looks. It's the total expected revenue per company member BEFORE the show has any expenses and usually it'll be low enough that you can say "OK, I'm not going to get paid for this gig." A lot of profit-share shows are sunk before they start because they're in small venues. In practice, it might look like this: You get invited to do a two-hander cabaret at a local 120-seat fringe venue. Two weekends, six shows. Calculations would look like this:
> 6 performances * 40 seats sold per show * $20 average per seat = $4800
>
> Four company members: two performers, one director, one SM/tech
>
>Therefore even with zero costs, you'll get no more than $1200 for the whole project
Take out costs (50-95% of most fringe/indie show budgets) and factor in how many days/weeks you spend in rehearsals. For this project, if we spent two weeks rehearsing/teching it and then had our six nights on stage, your fee could easily be <$600, or less than $37 per working day.
In all of this, I've assumed low attendance because (despite every producer's wish) most shows just don't sell that well. Several guides I've read, several producers and several fringe festivals agree you shouldn't budget on selling more than 30% of seats for an independent show, no matter how small (or large) your venue. Lots of producers will budget for 80-100% sales and it almost never happens.
Lots of exciting, important, or interesting shows never break even, and often great shows don't make a profit simply because the ratio of cost vs ticket price is so high. I spend most of my time working with opera, ballet companies, and orchestras, and they actually lose money on every show! All the full-time orchestras, operas, and ballets I know need private donors, corporate sponsors, and usually hefty government grants to stay afloat (some exceptions do exist for wholly commercial companies like the Heritage Orchestra, and some touring ballet companies, but these are very rare).
tl;dr: "Profit-share" usually means "There's no way this show can reasonably make enough money to pay you, so we're going to give everyone whatever we scrape away at the end."