Was the double negative on purpose there, or did you mean “if less than 51% are enforcing the new rules”?

There are just two cases:

• If a majority of the hashrate enforces the soft fork, the non-enforcing minority may occasionally find a block considered invalid by the majority, but the soft fork enforcing majority will ignore such a block and eventually build a heavier competing chaintip at which point the minority will reorganize to the majority chain.
• If only a minority of the hashrate enforces the soft fork, it will create a permanent fork as the minority will not accept the non-enforcing majority chaintip and the majority chaintip will accumulate more work faster. The majority chaintip will simply pull away and the enforcing miners will be stuck on a less-work chaintip.