What exactly does the strap do?

So I have some armwrestling experience but nothing crazy. Recently someone asked me why there's a massive difference with/without the strap. For context, he was armwrestling me and we're quite even on the right arm. Sometimes he'll win and sometimes I will win depending on whoever reacts faster. But with the strap, I can pin him much more easily like no problem at all. I told him the strap just prevents us from slipping and doesn't require us to hold onto our opponents as much. But is this correct? And what else does the strap really do? Why do they say the stronger person wins in the strap? I've seen scenarios where one guy will stone wall his opponent outside of the strap but once the strap is on, it becomes more balanced. Why is this?