OK let's say we have an item worth $10K with an effective life of 20 years.
Using the prime cost or straight line method you simply divide 100 (%) by 20, you get an answer of 5. This means you can claim 5% of $10K or $500 p.a. as a deduction for 20 years.
With the diminishing value method, firstly you use double the purchase cost, in this case it will be $20K, then double the depreciation rate to 10%. In the first year depreciation expense will be $2,000.
In year 2 the written down value of the item is now $8K, so depreciation expense is 10% of $16K = $1,600. The item's written down value is now $6,400 at the end of year 2.
In year 3 the written down value of the item is now $6,400K, so depreciation expense is 10% of $12,800 = $1,280. The item's written down value is now $5,120 at the end of year 3
In the straight line method, the depreciation expense is the same each year of usable life of the asset, in the diminishing value method the expense starts out high and then gets progressively smaller each year.
Hope that makes sense.