To jounce is to move up and down suddenly, the way you jounce around in the back seat of your brother's old car as he drives along a rutted dirt road.
When you jounce, you bounce abruptly. You may love to jounce down a frozen hill on an old plastic sled, while your best friend prefers the relative comfort of a huge, inflatable tube. Jounce is also a noun: "That last jounce knocked my teeth together." This word is probably a combination of bounce and jump or jolt, and we've been using it since the mid-15th century.