In wireless flash using a flash unit attached to the camera (either the built-in flash or a flash connected to the camera's hot shoe), there is NO WAY to prevent the flash attached to the camera to fire during exposure.
If you set the flash attached to the camera as control flash only, it still fires during exposure, but with less power.
One of the problems with the K7 (I don't know about the K5) is that when the built-in flash is set to control only, its power is more noticeable than the K10D and K20D.
I used to take photos of fish in aquariums (or is it aquaria
), so I became aware of this problem a few years back when I just bought my first K10D. I started then using flash with extension cord instead of wireless.
This applies not only to Pentax, but also for Canon, Sony, Nikon, ......
I think the strobe during exposure from the flash attached to the camera is to tell the remote flash to fire the main strobe. To verify this, set the camera to wireless flash mode, and set the timer to 2 second delay, you will see a strobe from the built-in flash during exposure, after the 2 second delay. It makes sense that the remote flash waits the (second) strobe from the built-in flash to fire. Let's say that the next Pentax DSLR has a 3 second instead of 2 second delay timer, it will work exactly as it does now without modifying the flash unit.
There have been many discussions on this. To be honest, I'm surprised to see this relatively new thread.