That didn't work, either :-(

The whole screen just scrolls.

What works for me is to not attempt to time it at all. Simply place two fingers directly on a message. Leave them there. Wait a second, then start sliding down. The message list should immediately shift to the right, exposing selection "circles" at the left edge of each message. As you continue sliding downward, each circle you "pass" will become selected. You can continue holding as you reach the bottom of the screen to keep the selection going. If you overshoot the end of the area you want to delete, you can slide back up to unselect a few rows before releasing. This works in both Mail and Messages.

Ha - on the iPad there is an edit button that exposes the selection circles; never occurred to me there would be a way to do the same thing on the iPhone. Nice!

I put two fingers on a message. The message shades slightly. I slide down. The shading goes away, and the screen slides down. It does not slide right, it does not collect $200.

When I put two fingers at the same time on a message, the checkboxes immediately appear. If I'm even slightly off, the checkboxes never appear and instead I get to see the contents of the message when I let go.

I tried this again and again.