It's easy how it works. Whatsapp considers a message as read once you have opened its conversation. WappStash takes the messages from the notification center, without using Whatsapp, so you can read all of them with privacy.
And even more, as you don't need to open WhatsApp to read your messages, you can also hide your last connection hour.
Forget about the airplane mode and some other tricks. You don't need them anymore.