Jack's not quite sure what it is that causes that sudden heat to rush through Ianto, but he's certainly not opposed to the results, feeling him come apart through every single point of connection between their bodies. And he's really not opposed to the way Ianto starts to slam into him like that, making the already overwhelming experience of getting fucked again by Ianto even more overwhelming. Overwhelming is exactly what he wants right now. He shudders and writhes and gasps under the force of those thrusts, clinging on for dear life, taking everything Ianto has to go give, lost and drowning in both physical pleasure and the churning emotions of being back with Ianto after everything that's happened.