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The principle is that you when you burn the supply of a coin the price will go up since there less coins availlable in the market ( Marketcap is the price of the coin multiplied by the circulating supply ) so if the circulating supply is a bit smaller but the demand is still the same so that drives the price up.
but here is good article I found explaining it more in detail.

Correct. Also in this case the SBD from the post is used to literally buy STEEM, so it is directly acting to increase the price (in addition to the indirect effect from fewer coins)

Coin Marketplace

STEEM 0.21
TRX 0.20
JST 0.034
BTC 90550.95
ETH 3108.45
USDT 1.00
SBD 2.92