Descongela literally means "melts" or "thaws." It's an interesting and evocative word because it implies a process of warming up something that was previously frozen or cold.
In the song, the lyric "Esa cintura descongela sentimientos" (That waist melts feelings) uses descongela metaphorically. It suggests that the person's captivating movements are so powerful they can thaw out previously hidden or cold emotions, making them accessible and warm again. This vivid imagery makes the word stand out.
The whole song hangs on one compliment, repeated until it stops being a compliment and turns into an argument: "Mami, te mueves muy bien" [Babe, you move so well]. Los Primos del Este build everything else around a guy who has been watching someone dance for too long and has decided this is love. "Por ella volví a creer en el amor otra vez" [Because of her I believed in love again] is a big claim to make about a waist, and the next line admits as much: "Esa cintura descongela sentimientos" [That waist thaws feelings].
The funny part is the position he is negotiating from. "Somos amigos pero se antojan tus besos" [We are friends but your kisses are tempting] puts him squarely in the friend zone, and his way out is to start listing things he can bring: a band to play under her window, "Lo que quieras de Prada" [Whatever you want from Prada], a photo of the two of them on her profile so everyone knows. He is not subtle about the rest of it either, asking for "Una noche no más, pa' que puedas probar" [Just one night, so that you can try]. The sax and accordion keep it light enough that the whole pitch reads as flirting rather than pleading.