Put simply, all you do it /ah sell <amount> in Survival with any item, then warp to Skyblock and type /ah, press the diamond, take back the items you want and then press the potato and bam, you get your items from survival and now have them in Skyblock. I'm not sure if this was intended but knowing Eric, Bosz, and Creator, I highly doubt this was an intended feature. In my opinion it's quite over powered and gives people a quick way to destroy their island levels.
This is a glitch Scottmg told me about, he mentioned that he wasn't going to report it to staff however, so I've decided to. Scottmg also mentioned that he himself had abused this glitch and that r2d58 has also abused this glitch a lot which maybe the reason it hasn't been reported yet... while I have no proof of Scottmg telling me this due to it being through voice chat, I'm sure Staff or Creator can just check their logs and reset their Skyblock or rollback everything they have done.