Thinking about taking the dive...

[adeyke]

That's correct. All four of the resistance-reducing skills (amp, decrepify, LR, and conviction) have the 1/5 penalty. If the monster is immune, the reduction is 1/5 the normal value, rounded down. If the monster isn't immune, there is no reduction. With LR and conviction, each resistance is checked individually (so a cold immune monster get only 1/5 the cold resistance reduction, but the other resistances would get the full reduction).

So a 100% physical resistant monster hit by amp would go down to 80%, while a 99% resistant one would go down to -1%.