Shadow Conjuration: Web - what would 20% effect entail?

[ksbsnowowl]

If a mage used shadow conjuration to duplicate Web, and the targets succeeded upon the will save, so that the spell only has 20% effect, what happens?

Would this be an instance of the spell not having anything to lessen as a percentage?  Otherwise, how would you have 20% entanglement?  Or the strength check to move?

[littha]

It only has a 20% chance of happening.

If the disbelieved attack has a special effect other than damage, that effect is only 20% likely to occur.

[ksbsnowowl]

Thanks.  I'm a little slow sometimes.