When you put a switchport into trunking mode, it will trunk and it will try to negotiate. Dynamic auto will wait for a request to trunk, so if the other side is set to trunk, a trunk will form. If you disable negotiating by issuing the switchport nonegotiate command, both sides will need to be set to trunk in order for a successful trunk to be created.
When both sides are configured to trunk, no negotiation is needed. A trunk will be formed based on the manual configuration of the switches and the switchports.