This is a very common practice on cheap van locks which don’t use an anti bump cylinder. All you need is a blank euro cylinder key which is then filed down into triangles, you then insert the key into the cylinder and leave a quarter on the key still exposed from the cylinder. You then need to tap the key with a small hammer this will make the tiny pins inside the cylinder all jump and the lock thinks the correct key has been used allowing you to open the door. A well oiled lock helps fight against key bumping but you should always look into having high security euro cylinders installed which are not subject able to key bumping.
The Thatcham approved van deadlocks we supply come with a flat drilled euro cylinder key which provides a very secure key set up and these cylinders fight against key bumping and lock snapping due to the way the are made. Anti bump and anti snap cylinders cost a huge amount more in terms of cost due to the high quality components used. Having a poor cylinder installed doesn’t give you an edge on vehicle security due to the key bumping technique being known by a large amount of people.