With 51% of the hashing power you can asymptotically do whatever you want with the recent transactions. Including erasing some of them and so allowing double spending, or blocking all the transactions. It&#x27;s much easier if you have more than the 51%.<p>With the 70% you can do more interesting stuff, like using a 35% to mine in the fork&#x2F;altcoin you like and use the other 35% to block the other fork&#x2F;altcoin so no one can make transactions in it.<p>With the 90% you can play games, like mining during the day as usual, and do fancy stuff at night. From 0am to 4am, keep a 10% as protection against attacks, and use the other 80% to mine empty blocks in the other chain, starting from the last block you mined last night. So in 4 hours with a 8x hashing power is enough to catch up with the other chain and then when they get up in the morning they will see all their transactions reverted.<p>A fork with little hash power is very insecure.