The primary record is retained with its values. You can choose the record (lead) that you want to set as the primary record. If the primary record has blank fields, then the values of the most recently updated secondary record is filled.

• Created at - The date of the oldest record (primary or secondary) created. 
• Updated at - The date when the leads are merged.The data from the primary record is retained

If the primary record has blank fields, then the data from the secondary records is filled in those fields. The secondary records are then deleted permanently.

Leads once merged cannot be split into their individual records.