The first half of the talk will be devoted to explain the fundamentals of this technique, stressing the difference between rigorous and non-rigorous methods. In the second half we will discuss applications to problems in incompressible fluids such as the confined Muskat, the $\alpha$-patches or the water waves problem. Some proofs will be presented in real time. Joint and ongoing work with Angel Castro, Diego Cordoba, Charles Fefferman, Francisco Gancedo, Rafael Granero-Belinchon, and Alberto Martin Zamora.