You’ve learned to put the weight of a weapon to your advantage, letting its momentum empower your strikes. When you successfully hit a creature with a heavy melee weapon attack on your turn, you can use your bonus action to make a single melee weapon attack against a different creature within 5 feet of you using the same weapon.
Great Weapon Master is not a bad feat. Actually, it’s very good. It’s possibly too good. This feat is designed as part of an effort to remove the -5/+10 mechanic from feats. The damage gained from these feats represent a huge amount of single target burst damage, making them a requirement to get as quickly as possible.
This modified version allows a bonus action attack every round, however the attack requires at least 2 creatures within 5 feet of you (or possibly 1 creature within 5 feet and another creature within 10+). This spreads out the damage, providing the great weapon master a reliable damage bonus, but does so spread out across multiple enemies.. The idea is that dealing damage three times feels really good but has far less burst potential, ensuring the character never gains upwards of 80 damage in one round from a single feat (Action Surge at 20th level). Compare 100 damage to 1 enemy, verses 20 damage to 5 enemies. While the second one has some use, the single large nuke is desired as creatures with high health are typically more dangerous.