The vacuum sealer uses heat to seal special bags. You re-seal the same way you seal it originally by placing the part of the bag with a corner cut off in the machine and turning it on. The bag will get a bit smaller each time, but even for long ferments I don't need to do it more than 1 or 2 times. I just make sure to size the bag originally to account for it.
Our vacuum sealer said to never use it on bags with liquids. I'm guessing that's just a stupid compliance rule in case something goes wrong and it sucks the water into the machine.
