Except IIRC some are equipped with a panel switch to do so during dumping..
Normally not!

A switch is also common in some older gliders notorious for sucking water into
the statics during dump (LS-3, LS-4 come to mind).

As Dave indicated, the LS-3 standard setup included a switch that pumped pitot pressure through the aft static ports to prevent water from entering while dumping. It wasn't strong enough to blow them out if they got full of water so you had to use the switch before you dumped to pressurized them slightly--and also for a some time after because water drained down the fuselage for a while. IIRC, the ASI used the forward statics but the open pitot line reduced the pressure enough that the ASI read low while this was going on. The switch also blocked off the line leading to the older Cambridge vario that used the aft statics. Since this vario had a capacity bottle, if you gained or lost much altitude while dumping (which was most of the time, of course), you had to switch off that vario to prevent pegging it out when the pressure in the capacity bottle was vented. All in all, it was a PIA, especially when getting low and contemplating dumping to remain aloft. Not to be repeated.

When I got my ASW 24, I (like very other U.S. owner) ignored the factory dump ports on the lower wing close to the fuselage (with small diameter electric valves) and installed BIG valves that dump through a hole in the bottom of the fuselage. I've never had water get in the aft statics.

Chip Bearden
JB