The connector on the FlexVolt tool is what enables the higher voltage of the pack. There are 2 things that need to happen, correctly, for the battery pack to output the higher voltage.
First the slide switch needs to be engaged.
Second, AFTER the slide switch has been engaged, the 54v tools connect C1 to C3, which enable the 54v mode.
WARNING: If you connect C1 to C3 before the slide switch has been engaged, it WILL damage the battery pack and ruin 2 of the cells in there. If you are going to use the handle from a 54v tool, which would be the best way to go about it, then you'll be fine as it will do all of that switching properly.
Also be aware that while it's running at 54v, it's going to be 1/3 the rated Amp hour, as that rating on there is for the 18v mode. So a 6.0Ah pack is only 6.0Ah at 18v, at 54v it's going to be a 2Ah pack.