The enhancer rolls a lower number on the drop table by 10%. This stacks with the 5% number reduction from completing the Elite Wilderness Tasks.[1]
References
^Jagex. Mod Pi's Twitter account. 13 April 2017. Mod Pi: "They work by lowering the number rolled on the table. Wildy diary reward is a 5% reduction. Drop enhancer is a 10% reduction.They stack."