Hi,
It's to lock the selector named VOICE. And it does so here, when having both the VOICE button and LOCK engaged like on your screenshot. With this settings it does'nt change to sample here, when changing presets. It stays on VOICE input.
If it does'nt work for you, then perhaps try to reinstall Vovoder V.
I you don't use the latest version, then you can try to update first.
I'm on Windows 10.