Currently, you can see the tooltip by hovering over either a revision bar or the greyish rectangle that extends it to the full height of the revision slider. If you hover first over the greyish rectangle, the bar is not highlighted. If you then hover over the bar, it gets highlighted, too. However, this new highlighting does not indicate any new option/selection...
Whenever the greyish rectangle is seen, the corresponding bar should be highlighted, too.
The revision slider is a feature that adds a visualization of all revisions to the compare screen.
The revision slider was originally prototyped by the WMF Community Tech team and then handed over to the WMDE TCB team, as it is also a solution to fulfill a wish of the German Community Wishlist: https://de.wikipedia.org/wiki/Wikipedia:Umfragen/Technische_W%C3%BCnsche_2015/Artikel#Anzeige_aller_Bearbeitungskommentare_im_Diff