OnlyIf commitments take the form "I'll do X ... but OnlyIf N other people will do it as well"
For examples see:
- FundableDotOrg - I'll pay X ... but OnlyIf N other people will pay X too.
- PledgeBank - I'll do X ... but OnlyIf N other people will do X with me.
- ConsensusPolling - I'll agree to commit myself to a solution to X ... but OnlyIf N other people help figure out the solution to X and agree to commit to it as well