To keep water out of a swimming pool's main drain line over winter, you blow the water until you get a nice big fountain (which requires that you use an air pump powerful enough to overcome the water column that you have) and immediately seal the valve. Similar to how no water will enter a straw that you insert in a glass after pinching, this will keep the water out of the main drain line.
To be extra-safe, it's best to not just blow the water out of the pipes, but to also pour anti-freeze.
Talking over a figure is best. (I've already used the storage at imgur.com, and so this is hardly wasteful.)
How do I use anti-freeze on a main drain line? It is simple enough with the skimmer line and the return lines (blow water out, seal the skimmer and the exit lines, pour anti-freeze, reconnect at pump to make sure no rodent enters).
It's far less clear with the main drain line. I understand that I need to blow the air out of the main drain line and then immediately close the valve. There is no window for pouring the anti-freeze.
Note for reference: There are apparently two kinds of anti-freeze. The one used in water-cooling systems in cars (methanol?) is unsafe to ingest, even in trace amounts. The other, plumping specific, anti-freeze (propylene glycol?) is safe to accidentally ingest in trace amounts (in a cottage after re-opening the pipes in spring, or in a swimming pool). The latter application is trickier. A farm house plumbing can be rinsed for a few minutes. No such rinsing is possible with a swimming pool. The anti-freeze just gets diluted. On the plus side, it does degrade and vanishes with time. I'm adding this important point here for the reference of others, but do comment if you find I got any detail wrong.