Its the same as with normal back orders only you will loose out to anybody who bid higher than you. I place 15$ bids for domains I want to catch but do not care about too much. 20$ bids on domains that are semi-important and I use the 59$ option for domains I know I will sell.
You may also want to try pheenix, they have a new 14$/backorder system in place sometime they do just as well as Dropcatch. You need to have at least $1000 spent with pheenix so you get the discount option.