When I was learning to tow, the best advice given to me was to let go of the line if the nose of the glider was pointed below horizontal while traveling to the side. If you don’t, it will only take a couple of seconds to turn and fly straight down into the ground. If you wait until it is actually heading down, you may freeze and need to pick up the pieces. This won’t happen after the model is overhead, but occurs if the line is more horizontal than vertical usually during the launch and climb phase where line tension is higher.
Also, to be sure you are launching straight into the wind, look at the towline between you and the model. If it is bowed to one side or the other, you are crosswind.