Coalescing Empty SPace in a Data Block

Does anyone know how/if you can force Oracle to colesce the empty space within a data block? We are able to coalesce empty blocks within a segment, but how does Oracle (specifically smon) know if/when to coalesce empty contiguous space within a data block?              

Regards,

Barry Sieben, OSG, ITSD          | Voice: (250) 387-9394
3Gr/E317, 4000 Seymour Place     | Fax:   (250) 387-5766
Victoria, BC, V8X 4S8            | IN: bjsieben_at_bcsc02.gov.bc.ca