Yeah, I was wondering about that, too......
If the starter is sitting there, "turned off" with the Bendix drive disengaged, no amount of "rocking" the vehicle in gear with the engine not running is gonna affect the starter, unless the Bendix is stuck engaged. If that's the case you should hear the starter turning over once the engine starts (it makes a funny noise). If the Bendix is NOT engaged or stuck and everything is "off", then the starter isn't connected to anything - it's just sitting there. When the key is turned to "start", the solenoid pushes a gear forward on the starter shaft to engage with gear teeth on the engine flywheel to start the engine. Once the engine starts and the key is released, the solenoid returns to rest and disengages the gear teeth so that the starter motor can stop turning.
(That whole mechanism was invented by Vincent Bendix, way back at the beginning of the 1900's.)
On the other hand, if absolutely NOTHING happens (no clicks, buzzes or pops), then it can probably be safe to say that the starter solenoid has had it and it's time for a new/rebuilt starter assembly (starter and solenoid come as a complete unit). If it clicks, once, when the key is turned to "Start" and nothing else happens, then you may have a "dead spot" or high resistance place on the starter armature. Whenever the starter motor stops turning on that spot it can't make electrical contact the next time you hit the key and nothing happens. It's random, too, and can be affected by trying it multiple times (sometimes you might overcome the resistance) or by vibration (like hitting the side of the starter with a hammer to jar it).
If, when you turn the key, the solenoid clicks rapidly (chatter) but the starter doesn't engage, that often means that you have either overall low voltage, or high resistance between the key switch and the solenoid such that there is insufficient energy going to the solenoid to make it fully engage.
If the builder used an original (donor) VW wiring harness, then there might be a voltage drop between the key switch and the solenoid which often causes the solenoid to buzz or click, rather than make a big click and engage the Bendix drive (this was very common with 6 volt systems - less common with 12 volt systems). THAT can be cured with a relay close to the solenoid (the key switch drives the relay, and the relay drives the solenoid).
Hope this helps.....
gn