The open bale-arm set up is used for predator fishing, this is because with species like Perch and Zander they need to be able to pick up a bait with minimal resistance.
The open bale arm set up allows the fish to collect the bait and then run with it pulling line directly from the spool.

This is achieved by placing an indicator on the back rod rest and putting the line directly beneath the spool into the clip (which should be set to the minimum strength). Then when the fish pulls, the line comes free from the clip.

This can be coupled with an electronic indicator such as the E.T. Backbiter (pictured) or a Ghetto Blaster type alarm.