Actually of double trebles / treble crochet, and some kind of it called in Polish "podwójny słupek reliefowy". I couldn't find an appropriate name nor in British neither in American English so I would be grateful if you share.
Well, the traycloth is being blocked at the moment and hopefully tommorow I'll be able to take good pictures.
Meanwhile, I'm going to show this less popular double treble ;)
"the tricky one" ;)
And the way to lead the hook
The right version : you lead the hook conversely to typical treble (from back of the stitch instead of front) but before you grab the floss - you need to put the hook through the next stitch in regular way, from the front.
As a result you get the sunken treble.
The left version : you lead the hook similiar to typical treble, but before you grab the floss you need to "come back" through the next stitch.
As a result you get the convex treble.