Originally posted by Chip
Yes that would be correct if you had a chamber full of water. However, hydrolock and engine damage can and does happen when just a small amount (since it is uncompressible) causes a extreme peak in pressure. So with this spark plug it reacts before you get that far. Any pressure relief you can give to any pressure vessel is a good thing since you will not have those damaging peak pressures.
That is always the thinking when talking about hydrolock, that a small hole could not escape alot of water, but the case being as stated above that when you get just a little bit that would be damaging (doesn't take much since water does not compress, and as we have seen in testing) it can escape and vent the pressure that would otherwise cause damage.
We really appreciate the comments and questions.