Affect's Engine is an immersive ethnography of queer people of color on the social media platform Tumblr during its "peak," from 2010 to 2015. Chronicling a formative time in social media history, Alexander Cho traces the affective patterns and practices of these users, showing how they forged deeply felt, often wordless connections while also cultivating a vibrant politics of resistance to white supremacy and heteronormativity. From "intersensuality" to "attunement" to "haptic wayfinding," Cho offers a vocabulary to describe these cultures of use that travels across media studies, affect studies, human-computer interaction, and queer of color critique. By paying critical attention to Tumblr's unique user experience, he argues that the same features that drew queer people of color to Tumblr to feel intensely also explain why the platform struggled to succeed financially: its design did not cohere around the good liberal subject-linear, singular, discrete, orderly, public, and market legible. Cho paints a vivid picture of a vital and bygone internet era, and asks readers to take seriously how affect is shaped by user-facing design on social media.