An explanation from an anonymous conductor:
What’s with pointing up when you stop at a station?
We’re pointing at the conductor’s indication board, which is a zebra-striped sign. If the sign is in front of my window, it means that the entire train is on the platform. They don’t trust us to just look (see that other question about zoning out), so required procedure is to point to it at every station before we open the doors. The absolute biggest violation a conductor can make is opening the doors where there isn’t a platform. If that ever happens, the first thing supervision is going to ask you is, “Did you point to the board?”
Or, as that charming video suggests, they could be pantless.