"Why" is because it's fun.
"How" is a tougher matter. I've tried before with one partner and was never successful. I've tried with another partner and have been successful. The key tips are:
- lots of lube
- go slowly
- scrunch fingers (and knuckles) together as much as possible
- lots of lube
- short fingernails
- lots of lube
Good luck to ya.
