BitShares SmartCoins are pegged to their counterpart indirectly in the same way that SBD is pegged to USD. The peg is based on the guarantee that a smartcoin can be exchanged for an equivalent amount of the backing asset. I. e. 1 SBD can be exchanged for 1 USD worth of STEEM, 1 bitUSD can be exchanged for 1 USD worth of BTS (BitShares).

When the proposed parameter change comes into effect, 1 bitUSD can only be exchanged for 0.99 USD worth of BTS. That means that effectively bitUSD is not longer pegged to USD but to 99 US-cents instead, which means a devaluation of 1%.

Note that the modification of the settlement "guarantee" does not only affect the settlement as such. It will be priced in by market participants, which means that bitUSD will also trade 1% lower on average.