These increase the effective length of your lead - a 30cm (12") couple attached to a 1m (40") lead will have a total legth of 1.3m (52")
The maximum distance beteen the two dogs is the combined length of the arms - a 30cm (12") couple will allow a maximum of 60cm (24") between the dogs
Large dogs need longer couples than small dogs
If the couple is too long, it may drag on the ground and get caught around the dogs' legs (or yours)