What works for me is to keep the slave in such a sense of arousal/subspace where they're able to keep swallowing. For most the reality overwhelms them and then the gagging starts. Once that starts a mental reset is needed. Then ball squeezing is one of my go-to's to get the slave to keep eating. Or aggressive pinwheel, nipple twisting followed by a break where the slave can face another 5-10 minutes of pain, or resume eating my waste.