The ADSR envelopes cannot be modulated in such a way that you go back and forth between sections. Once the Attack phase has completed, you are stuck in the decay phase and cannot go back. This means that one-time modulations such as velocity->attack are very effective as they lock the attack time to a value, but LFOs or recursive mods are not. Example:
Attack is set to a short value. You wish to modulate the attack to get a bit of a tremolo effect. If you modulate the attack time, you may expect it to shorten and lengthen and cause an LFO type effect on the envelope, but this isn't the case. If you set an LFO on the attack, the attack is calculated with the LFO until you hit a point which is past the end of the current attack phase... so if the LFO modifying the attack happens to cause the attack to go to 0 for even a single MOMENT... the attack phase ends immediately and the decay phase starts. This has a very noticeable effect on the envelopes when you try and create the recursive behavior we suggest to create envelope curves. If you use the envelope to modify the attack time, you get a curve that is only effective over a small range of the mod.
There is a good workaround to modulating the value of the envelope during the decay phase... set the modulator to the sustain and it will control the end point of the decay phase without changing the length of the decay. We can create a similar effect on the attack phase by modulating the amount of the envelope, as it changes the end point of the attack without changing the length of the attack. The problem is in both cases, the modulation continues past the end of the phase (modulating the amount of the envelope affects it during the entire period of the envelope) so this limits the usefulness if you're trying to create an effect with a precise response that ends in the middle of the envelope.
To create a gated effect, the easiest and most controllable way to do this is with an aux envelopes. The key is the two secondary envelope parameters: delay and peak hold. Delay allows you to set a period of time before the envelope starts and peak hold allows you to set a period of time the envelope is held at the max value without worrying about note on/off behavior. To modify the attack behavior of an envelope, set the peak hold of the aux envelope to roughly the value of the attack of the target envelope (use your ear to get close as the values are not 1 to 1 between delay/peak times) and set the destination of the aux envelope to the amount of the effect you want on the attack.
Using the tremolo example, first find the amount of tremolo you want using LFO1 applied to the VCA envelope amount. Once it sounds right, note the amount of LFO1 (let's say 40) and set the amount to 0 so we can modulate it cleanly. Now we set up our aux1 envelope peak hold to roughly the amount of the VCA attack and the aux 1 attack to 0, decay to 0, and release to 0, giving us a gate effect. (you can smooth the effect coming in and out by increasing A/D/R). Now assign the destination of the aux1 envelope to LFO1 amount and the amount of the aux1 envelope to the value you liked on the tremolo (from above, 40). Now you have a tremolo effect that is applied only during the attack phase of the VCA envelope.
You can change this to an effect only on the decay portion of an envelope by using the delay param of aux1. Using the above example, by setting the delay of aux1 to roughly the time of the attack portion of the envelope and setting the peak hold to the decay time of the envelope, you would be adding the tremolo over the decay phase.