How to swallow dildo in ass hands free

Hi folks.
Just a quick post.
I have seen a few videos of women and men that can suck a dildo in all the way into their ass and swallow it whole hands free.
Can anyone here do it. I wish to learn the techniques on how to do it.
Seem you first need to insert it initially, a few inches, and then your ass takes over and pulls/sucks it in all the way in.
Looks fucking hot. Bet it feels even better once the technique is perfected.ANy tips would be much appreciated.
Thanks in advance.