You don't have to time anything. Just KEEP pressing down with your finger/stylus/implement-of-choice and then slide/drag your finger across the menu.

So:
Press.
Keep pressing as menu pops up.
Without decreasing the pressure (at least to the point that it thinks you stopped pressing) slide your finger to the menu item of choice.

Yes, it will "select" whatever option happens to pop up under your finger, but every program I've used on the N900 doesn't actually respond to the selection until you let go. So, until you let go, you're not selecting (as in pressing) anything.

Which is still a bit annoying, because sometimes it thinks you let go when you ease up on the pressure, and/or when your finger blocks the menu that pops up.