when the observer block is facing water any ajacent block, encluding up when a redstone component is broken/powered/points a different direction the observer block will send a quick and rapid pulse.
thus when a redstone dust is behind it and is broken and quickly placed back/powered/points a different direction it will send a pulse and then since it was powered once again a pulse will be sent which means it would be a fast rapid clock

when the observer block is facing lava any ajacent block, encluding up when a redstone component is broken/powered/points a different direction the observer block will send a slow and steady pulse.
thus when a redstone dust is behind it and is broken and quickly placed back/powered/points a different direction it will send a pulse and then since it was powered once again a pulse will be sent which means it would be a slow steady clock

also as far as I can see this works with redstone dust, redstone torch, repeater and comparator*

*_emphasized text_comperators activates this when placed/broken/only when goes from subtract to compare mode/powered and only activates it if it facing into it or the oppisite of the observer block.