The term "trench composting" pretty much says what it does. Dig a trench, shovel in some kitchen compost in varying stages of decomposition, fill in the trench then let the soil digest it for a few months. This method suits crops that like to root into rich, moist earth e.g. broad beans and runner beans.
 There's a lot to be said for the "no dig" approach i.e. building soil structure and fertility by regular mulching. In the case of this bed I had already dug it over thoroughly to remove a considerable amount of couch grass. Even Charles Dowding the guru of no dig advocates removing couch grass before instituting a no dig approach. So some trench composting seemed in order to get things underway.