vco pulling pushing

Pushing refers to a change in VCO frequency due to a change in the power supply voltage. Pulling refers to a change in VCO frequency due to a change in the load on the VCO output.

Pulling effect take place when the synthesizer is trying to lock the loop as the voltage is still varying. Pushing figure take place when you connect VCO to other circuit which is not matched to VCO impedance. The effect is critical if you use frequency synthesizer with VCO as LO to modulator or mixer (where the impedance is normally not 50ohm).

I think toonafishy's answer is the right one.
regards

vfone: Toonafishy explanation is just simple defination for pulling and pushing figure. To better understand the effect of both figure in actual case, let say you are designing a PLL frequency synthesizer with VCO, the effect will take place (as what I have explained earlier). This is why designer always put 10-20 dB of attenuator (match to 50ohm) right after VCO to ensure VCO does not "pulled" if load is not 50ohm, followed by and buffer amplifier to provide sufficient isolation.

activewei
Document below is written by one of the biggest manufacturer of VCO’s in the world.
http://www.minicircuits.com/appnote/vco15-15.pdf
If you read carefully you will see, as I said, toonafishy’s answer is just to the point.
Regards