The transmission effect of miner selling pressure on small - and medium - cap altcoin prices is significant. Miners need to sell their mined coins to cover costs and make profits. When the market is weak, their selling activities can increase the supply of coins in the market, putting downward pressure on prices. Small - and medium - cap altcoins are particularly vulnerable as they usually have lower market capitalizations and less liquidity. This can lead to a vicious cycle where falling prices further discourage investors and increase the selling pressure from miners.