The problem
Most old houseplant guides suggest adding a layer of gravel or stones to the bottom of the pot before adding compost. It is presented as basic good practice; the thing you do to stop soil from retaining water, which can cause root rot.

The hack
This layer of gravel is said to improve drainage by providing a place for excess water to collect below the root zone, keeping roots above the waterlogged area and allowing air to reach them from beneath.

The method
Cover the drainage hole with a layer of stones or gravel before filling the pot with compost, then plant as normal. The gravel layer will fill the bottom quarter of the pot, reducing the available root space before you even add soil.

The test
I set up two pots, one with a gravel layer and one without, and watered both. Pressing a finger into the compost just above the gravel in the first pot, the soil was wetter than in the pot without it.

The verdict
The science behind this is the perched-water-table effect. Water moving through fine compost will not drain freely into coarser gravel beneath it until the soil above is completely saturated. The gravel raises the waterlogged zone, bringing it closer to the roots. If drainage is the goal, the answer is a pot with adequate holes and free-draining soil. Leave the gravel for the path.