I also wanted to demonstrate the difference between prepped and unprepped flats, so I have 2 videos here. They both show me pouring 1 cup of water on an unbleached flat. In the first video, the flat is unprepped - exactly the way I brought it home from the store. The second video shows the same amount of water being poured onto an unbleached flat that has been boiled on the stove.
You can see how the water beads up on the flat and takes time to absorb into the fabric. The boiling removes the natural oils (be sure to do a hot rinse while still hot or some of those oils can redeposit back into the fabric as the water cools.
I would expect that the flats will continue to become more absorbent for the next few washes, but I actually used them just like this, and we have not experienced any leaks, so they are certainly adequate after boiling once.
If you have leaks and suspect that your diapers might be repelling, you can always try a kitchen experiment like this to find out how well the diapers are soaking up liquid.