How did Eren transform into a Titan without causing self-injury (Season 4, Ep. 5)?


So at the end of Season 4, Episode 5, we see Eren becomes a Titan while
talking to Reiner. He didn't cause any self-harm, so how did he transform?
Should it be assumed that they just didn't show him causing self-injury?

Any answer is appreciated :)




you see his hand is already bleeding before he shakes reiner's hand and goes
boom. he was just waiting to activate his power[![enter image description
here](https://i.stack.imgur.com/skmci.png)](https://i.stack.imgur.com/skmci.png)




you see his hand is already bleeding before he shakes reiner's hand and goes
boom. he was just waiting to activate his power[![enter image description
here](https://i.stack.imgur.com/skmci.png)](https://i.stack.imgur.com/skmci.png)

