How much traveling are you planning in the area covered by the B.O. pass? Have you seen to coverage map? If not, CLICK HERE to see it. Only the lines in red are fully covered. Its not a lot.
I haven't used this particular pass I think the site is suggesting that you buy a 50% separate ticket from Lucerne to a point where your pass will fully cover and then use your pass. It looks confusing to me as well.
You might look into the Swiss Flexi-Pass which would completely cover you from Lucerne to most anyplace in Switzerland on a covered day. Also, if most of your travel is in or around the Interlaken/Lauterbrunnen Valley, you might find that 50% on the local fares is just fine instead of using a fully covered day.
Please don't go for any Eurail pass. It will only get you a 25% discount in the entire Lauterbrunnen Valley.