My understanding is they keep the mast straight (in column) in heavy air. This preserves the correct mainsail shape. There are some pictures and drawings of the in-line lower setup in the deck layout and rigging photo gallery. The easiest way to install in-line lowers is to call Buzz Ballenger at Ballenger Spars in Santa Cruz (Watsonville?). He will provide you a kit and instructions for a reasonable price, if you decide to do it.