Insoles with a rigid arch support will flex extremely little under pressure. If you imagine holding the insole in both hands, with one hand on the midfoot of the insole and the other on the heel, if you were to try and bend the arch of the insole it would be extremely difficult to do so and the insole would flex very little. This type of arch support is effective for providing maximum foot support, but will provide very little flexibility during activities such as running. Insoles with a flexible arch support are essentially the opposite. This type of arch support will freely flex under pressure. If you again imagine holding the insole in both hands, if you were to bend the insole, the insole would freely bend over into itself. This type of arch support is great for providing gentle support and overall cushioning for the foot and arch, but may not provide enough support for conditions such as plantar fasciitis. Semi-rigid arch support insoles are between the other two options. They will flex under pressure, but will resist fully bending. This arch support type is ideal for most situations, as the support will resist flattening under pressure and will keep the foot and arch supported, while at the same time also allowing your foot to flex more freely.