Not sure if Thomas is already typing as I'm responding here but what he said is right on. I'd add two more points:
1. Dry the paper before looking at the test strip
2. Once you've got the flash time figured out, you must flash the paper before you do all your further test strips, work prints etc with the negative.
There is a certain minimum threshold exposure required to overcome the "inertia" of the emulsion in the paper before any tone is produced. Pre-flashing is generally used to give the paper just enough pre-exposure to get to that threshold. The idea here is that any additional exposure when printing the negative will produce tone. This can make it a little easier to get some tone in dense highlights without having much effect on the darker tones. It is sometimes useful, although less useful than it once was given current variable contrast papers. Note a few things:
1. Pre-flashing WILL reduce local contrast in highlights
2. Pre-flashing WILL have a subtle local contrast effect extending further down into the midtones than often expected. This can be mitigated to some extent by backing off a little on the flash exposure instead of using the maximum time indicated by your pre-flash test strip.