I think it works as follows: say you make sales for one million plus 20% VAT. You get paid 1.2mn, but 200k are for the taxman. However, you already paid VAT for your inputs, say 30k for 150k in materials and 20k for 100k in electricity. You then give to the state only 150k: the 200k you collected minus the 50k you paid (i.e 20% of the 'Value added' 1mn-150k-100k).