Dragons number eight Ross Moriarty received a yellow card for a late hit on Leinster fly-half Johnny Sexton.
The Ireland pivot was putting on a clinic in the first-half at the RDS and was orchestrating another move down the Dragons left hand side when Moriarty hit him after the pass.
Moriarty appeared to be taken aback by his temporary dismissal but the footage clearly shows that he wasn't exactly subtle in his timing.
After Sean Cronin opened the scoring following a beautiful switch with Jamison Gibson-Park, Leinster tagged on the scores following Moriarty's sin binning with Josh van der Flier, Gibson-Park and Jordan Larmour all notching further tries as Leinster look like they will coast their way clear to a comfortable bonus point win.
A win for Leo Cullen's side would help the defending champions close the gap on conference leaders Ulster, who play on Sunday, while Treviso, the only other unbeaten team in conference B, currently trail the Scarlets at the time of writing.
