Another way to visualize it might be to think of a send as a new, separate track. With an insert, the effect you apply is coming through the track you applied it to, so if you turn up the gain or pan the track all the way to the left, the effect is going to respond in kind. This is why you want to use "replacement" effects like compression as inserts. You want your insert to take the place of the original sound on the track. With a send, you are literally sending the signal from one track to another. The original stays where it is, and responds as normal to panning, gain adjustments, etc... However, that sound is also now being fed through some effect (like reverb) in another track which can be panned or gain adjusted completely independently from the original track.
This situation I just described is a pre-fader send. If you've been messing around with sends in your DAW, you've probably seen some option for pre vs. post or something along those lines, and that's what it means. If you want the sent signal completely disconnected from the original, makes sure it's pre-fader.
One more thing to keep in mind to help you visualize it:
A lot of vst (or AU or whatever) effects have a mix knob or slider or two, where you can decide how much dry (unaffected) versus wet signal you want. Generally, if you are using an effect as an insert and you want to replace (like with a compressor), you would want a completely wet signal. However, if you are using an effect to change instead of replace (like reverb) and you are using it as an insert, you'd want mostly dry signal, and then you'd dial in the wet. However, any time you use an effect as a send, start off with it 100% wet. Then use your gain and pan sliders to position it around the original unaffected signal.