Hot trans fluid pumping through the cooler lines under pressure.
Trans coolers are designed to contract when cold to help heat the fluid quicker. They then expand when hot and allow normal flow. Blowing air or solvent through them cold will not dislodge crap in the cooler. When the trans fluid gets hot and the cooler tubes finaly expand, the crap comes out. Many times it's too late and trans valves and solenoids will act up causing problems and premature failure. Hot flush machines are a must. The one we use here at the shop costs over 6K. Money well spent and it paid for itself many times over.