I've contacted support, but info received was insufficient
Anyway, what I guess:
the two modes of operation may be chosen, with 'disabled switching"
and "enabled switching"
It is a user configurable, depending whether or not two or more ports are "switched" together.
There is a dependency on the version of RouterOS though:
* pre 6.41: if at least one port is slave to a master -> "enabled switching" applies
* post 6.41: if at least two ports are bridged in hw(=yes) -> "enabled switching" applies
* otherwise "disabled switching" is effective.