It is the Jayce Champion detail. Jayce is equipped with an interesting ability consisting of Thundering Blow + Transform Mercury Cannon. He can knock back somebody hostile and change his ranged form in a wink so as to obtain the max attack speed for some hits.

According to Teamfight Tactics Jayce guide, he is regarded as the sixth piece in the Transform group. He can turn the Thundering Blow into Cannon to snip the foes. Aside from that, Jayce will gain enough energy before he becomes different. However, he can be destroyed when he stands at the frontline.