GSN without signing?

According to FAQ page about GSN, It is stated that:

An example would be an application which requires users to register via a normal (non-gsn) Ethereum transaction and then for some time afterward, they would not be required to use Metamask to sign each transaction, rather the transactions would be handled by the GSN relays. This could be very convenient for applications which require a high number of transactions and where a confirmation popup for each transaction would be distracting or give a poor user experience.

As far as I know, user needs to sign each request using Metamask to get it relayed, so for each transaction the Metamask still needs to be popup. So this contradicts the last paragraph quoted.
How is it possible for the relayer to relay a tx without the tx being signed by the user?