Exactly, you have understood very well. 😀
On the eleventh day this ten-day timeframe which the Land Registry gives you to protect you interests ceases, and anyone who comes along and registers a change in the ownership of that property and registers it before you will have priority over you despite you completing on it before them at a Notary.
To avoid such a stupid legal loophole it is very advisable to have your NIE number ready before completion, so that we lawyers can arrange everything within those ten days “protection” Land Registry days after completion.
Lex dura, sed Lex.