Backorder
A standing request to register a specific domain the moment it becomes available. Backorder services queue up at expiration windows and compete with each other for the drop.
Cost
Free to place (most services). Charged only if the catch succeeds, typically $60-$80. Higher value names go to a private auction among multiple successful catchers.
Top services
DropCatch, SnapNames, NameJet — each runs their own catching infrastructure with different success rates by TLD.