Assume a system with 4 AMPs and a database with 1 GB of temporary space. The per-AMP quota of temporary space is 250 megabytes. If you set the temporary skew limit to 20%, any AMP is allowed a skew limit of 50MB above the per-AMP quota of temporary space. An AMP can use up to 300 megabytes of temporary space if total temporary space used does not exceed the 1 GB global limit for temporary space.
MODIFY DATABASE d1 AS PERM = 1e9, TEMPORARY = 1e9 SKEW = 20 PERCENT;