ASICBoost is a technique for mining more efficiently (something on the order of ~20%, which is huge if margins are thin). There has been a lot of controversy over allegations that Bitmain has been using a form known as covert ASICBoost that is difficult to detect, in their own mining operations. Their hardware was built for ASICBoost, but they did not enable it for customers. That and some quirky behavior (mining empty blocks and championing their right to do so at a time when fees were skyrocketing due to congestion) led to suspicion they were rigging the mining game to their own advantage.
Noteworthy is that covert ASICBoost was incompatible with Segwit, and Bitmain was fiercely opposed to Segwit, holding up the critical scaling technology (and thus the Lightning Network) for months for reasons difficult to understand if they weren't using covert ASICBoost. They split off and created Bcash (which retains covert ASICBoost as an option so they could presumably continue milking their cash cow) when it became clear they couldn't stop Bitcoin from scaling with Segwit/LN.
Overt ASICBoost is now leveling the playing field as other miners take advantage and do so above board and transparently
.more info:
https://blog.bitmex.com/empty-block-data-by-mining-pool/
