Pitch over time with Portamento Start set to PITCH. C4 is played first, then C5, so the pitch glides up toward C5. Before it gets there, D4 is played, and the pitch turns toward D4 from wherever it was at that moment.
Compare Portamento Start: NOTE.
Diagram from the XP-80 Owner’s Manual, © 1996 Roland Corporation, reproduced for educational purposes. Credits & disclaimer.