Find a way that suits you to let anyone behind know that you are aware of them.
If I come up fast behind another bike,I can tell when they've noticed me,and in the same way I can let a faster bike know I'm aware of them. You don't need waves,turn signals,fireworks,whatever, it's just body language, bike positioning etc.
If someone waves me past,fine,but I'll pass when I'm ready and if someone's about to pass me then I'll try make it easier = safer, for them.
Always look after No.1
Ride safe.