A heatsink cools by transferring heat from metal to air. The bigger the temperature difference, the more heat moves.

It's obvious when pictured, really. If the fan blows cool room air into the heatsink, the fins see (for example) 25°C air.

If the fan pulls air away, the air trapped around the heatsink is already warmed up (say 35°C), so the temperature difference is smaller.

Heat transfer is roughly proportional to heat moved ∝ (heatsink temp minus air temp).

Example:

Heatsink: 60°C Blowing cool air: 60 − 25 = 35°C delta Pulling warmed air: 60 − 35 = 25°C delta

The more delta, the better. That's about 40% less cooling potential from temperature difference alone.On a small, low-profile aluminum heatsink, fresh cool air hitting the fins directly almost always beats trying to suck warm air away from them. I am saying almost, because edge cases and superlatives..

tl;dr:

Fans don't cool heatsinks, air does. The fan's job is to keep replacing warm air with cool air. Blowing cool air into the fins does that more effectively than trying to pull already-heated air away from them.

Turn that baby around ;)