I will attempt to define parts of the z drag for our discussion.
let's start at the object we wish to move(pinned boat or log, etc). attach the rope to the log and run it to pulley A, which is attached at an Anchor.
now run the rope back towards the object. somewhere on this run is where we attach Pulley B. most folks use a prussik here and I think this is good. it would be possible to use a tibloc and it does have it's merits, namely it is very fast to set up and quite easy. I still like a prussik cord and the Klemheist knot.(a digression here. for those who don't know this variation of the basic prusik. it is easier and faster than the standard prusik knot to tie and easier to use. it's drawback is that it can only be pulled in one direction but for this application we only need to pull in one direction. therefore i find it superior.)
okay so we have the rope running from the object towards the anchor and pulley A. back towards object to a prussik and pulley B, no back towards the anchor where we are holding and heaving. now once we have pulled our prusik to us we must Brake the system so that we can release tension an slide our prussik back towards the object . this is where the Tibloc really comes in handy. it is possible to use a second prusik loop as your brake but this requires continuously tending (sliding) the Braking prusik. not so easy to do while pulling hard. the Tibloc can be quickly and easily applied as the system Brake and in my opinion this is the best application for this tool. they are a bit pricey especially when compared to cheap prussik cord but as you can see from the rest of this thread, the folks who like them really like them.
I hope this is helpful and not too confusing. If I have f-ed up the description of the system i apologize for the increased confusion this may produce.