How do you know when to increase difficulty in obstacle training?

The correct time to increase difficulty in obstacle training is when the horse can complete the current level of challenge calmly, repeatedly, and without rushing — not when the horse has merely gotten through it once, and not when a fixed amount of time has passed. All three of those criteria matter equally: calmly means the horse's body and emotional state during the challenge show genuine relaxation rather than suppressed tension, repeatedly means the response is consistent across multiple attempts in a session and carries over to subsequent sessions rather than being a single successful attempt followed by regression, and without rushing means the horse is moving through the obstacle at a deliberate and controlled pace rather than getting through it as quickly as possible to exit a situation it finds uncomfortable. A horse that completes a tarp crossing but does so in three rushed strides rather than deliberate steps has not yet reached a calmness level that supports increasing the difficulty. A horse that walked through a tarp quietly in yesterday's session but shows tension on the approach in today's session has not yet consolidated the learning enough to move forward. Tension when the difficulty increases is the most reliable signal that the previous level had not been truly confirmed: a horse genuinely ready for the next step almost always handles the increase in difficulty with mild curiosity rather than significant anxiety. When the increase produces anxiety, the training should step back immediately rather than pushing through — continuing at a level that is producing anxiety trains anxiety rather than confidence. The pace of progression through difficulty levels is entirely dictated by the individual horse, and the horse that progresses slowly but solidly through each stage arrives at the hardest obstacles with a depth of confidence that the horse pushed through each stage too quickly does not have.