I know there is a trick which consist in hitting with the car on the side of the checkpoint,

that doesn't work for me, I supose it's because of the car I am using.

With which cars does it work?

Here's a checkpoint trigger collider so that you know where it's triggered, hope this helps.

Watch some of the Pink Plains any% runs, they do it on the second cp. Send me your discord and I can send you some clips of smooth cp hits.