Essentially pre-sales with bundled access to beta (or earlier) builds, early access is a common mid-development option. If primary funding falls short, it can help bridge the final gap.
Ranked in these QuestionsQuestion Ranking
Pro Get end-user feedback earlier in development
Because early access doubles as a public playtest, you can get valuable user feedback while there's still time to implement changes.
Con Not viable until the game is in a playable state
Early access depends on consumer interest, and few consumers will be interested in an unplayable pre-alpha build. Another option must be used for early development.