You need to flush chache since every host uses its own RAM to cache ios. However before splitting a BCV you need to have a "consistent" image thus you flush cache to disk
In other OS this operation is usually achieved via "sync" command. Since WinNT didn't offer a CLI utility to flush cache to disk, EMC included symntctl to allow flushing cache to disk.

Digging a little, I found that with NW you have to dismount the filesystem before splitting BCV devices.. however if you find a way to flush cache, maybe it may work without unmounting the device...

Deactivate the STD volumes (by a dismount and NSS /DEACTIVATE= volume) statement) so that cache will flush to disk.

I think you have to find an equivalent NW command (similar to the "sync" command)

Oh BTW neither symntctl nor "sync" command will ever pause IO against disks ..
If you want to suspend IOs against disk devices you have to unmount (in unix terms) or dismount (if you prefer) devices.