When equipped, it will provide infinite water runes similar to an elemental staff. It will also attempt to "save" runes used in any spell. This is how it does so:
The trident finds out how many different rune types the spell requires.
For each rune type the trident makes a 1 in 30 roll.
If successful it will save all runes of that type.
For example, the spell Earth Blast requires 3 air runes and 3 earth runes; there is a 1 in 30 chance that once cast, the penance trident will save either 3 air runes or 3 earth runes. The penance trident cannot save only some of the runes of any rune type; it will either save none or all of any rune type.
This rune saving ability is not as cost-effective as using an elemental staff in most cases, but a number of spells do not use any elemental runes besides water. Elemental staves, with the exception of the Armadyl battlestaff and Camel staff are also low-levelled compared to a Penance Trident.
Of course, for some spells, the 1/30 saving might outweigh the cost of elemental runes, but the Grand Exchange prices change such that this is never permanent. The rune saving ability works for all spells. For combat spells, the staff of light may be a better choice since it can save all of the runes for a combat spell with 1/8 probability.