From the release "..You can try out a preview version of Magistral Medium in Le Chat..", I suppose it's when the drop down in "Thinking" mode is either slow or fast (limited to 3 queries per day).

My favorite from the last months was asking for a string that for base64 produces strings with non-alphanumeric and non-padding symbols (so '+' or '/' should be in the output). It thought for 7 minutes and 74k of markdown length, and finally came up with the AB?C string that produces QUI/Qw== (correct). It is impressive, because general LLMs just always fail, but I didn't try other "thinking" models recently.