Just like if you publish your code as MIT or BSD. It will still require attribution, and they still can't re-license it to something incompatible with the original license. Turns out that closed-source is compatible with MIT/BSD (still requiring attribution).
And the "closed-source" part will be only the new code added after your contribution, but the project itself will keep the open source license (so that you can fork it at the state it was when the authors decided to go closed-source).