The angle of the lozenge. The lozenge in the starter bit is at 90deg, the lozenge in the verbindend is not. With the verbindend aimed to act on different pressure points in more established horses. It's described in more detail in enlightened bitting booklet
I use a verbidend on G horse. The mouthpieces are different shapes too. The starter is a more gentle curve with edges,whereas the verbidend almost curves inwards on the arms. The Verbidend is good for horses with a small amount of room in their mouths (fat tongue or in G's case too many teeth