press the button then move to left or right and hold the button pressed for a moment. release the button during the jump or still pressed if you want to make more jumps in a row. for example to move fast in long tunnels or places where a lot skulls rolling around.
Watch my gameplay in the video above. I play very fast using the jump. I prefer to play in Vice and 'allow keyset joysticks" so I play with keyboard only. Much more precise than a joystick. I play in general every C64 game with keyboard. ofc as a kid I loved the joystick but nowadays I can't imagine to play with joystick. hahaha