The occurrence of reverse flow in a channel when an obstruction is placed at the entry is already known. In this article an attempt is made to bring out the mechanism that governs this phenomenon. Flow visualisation and pressure measurements are employed for this purpose. The obstruction at the front end of the channel is essential for triggering the reverse flow. For reverse, stagnant and slow forward flow, sharp changes in presence occur at the two ends of the channel. The resulting flow field gives rise to a delicate balance of pressure along the channel that controls the flow direction and magnitude. It is argued that there is a limitation on the maximum reverse flow achievable.The occurrence of reverse flow in a channel when an obstruction is placed at the entry is already known. In this article an attempt is made to bring out the mechanism that governs this phenomenon. Flow visualization and pressure measurements are employed for this purpose. The obstruction at the front end of the channel is essential for triggering the reverse flow. For reverse, stagnant and slow forward flow, sharp changes in pressure occur at the two ends of the channel. The resulting flow field gives rise to a delicate balance of pressure along the channel that controls the flow direction and magnitude. It is argued that there is a limitation on the maximum reverse flow achievable.