There typically isn't an ON/OFF feature in effects that can be assigned as destinations. Some effects - particularly the multi-effects - do have this option. But generally, you'll be dealing with the levels of the effects and using those as a way to turn the effects "on" or "off".

1) Recall stock "Hybrid EP" preset Performance
2) Mill around and look for the phaser effect - First, I'd look at InsA and InsB of PART 1 (the only PART) by looking at the Effects "Routing" page. I don't see it there. I see "G Chorus". Then I look at system effects by pressing the Performance name, Choose Edit, then select Effects routing there and see there is no phaser effect here either. The delay is in the system effects (variation = Tempo Cross Delay).

Are you sure you're looking for Phaser? Or is it the "G Chorus" you're calling the phaser?

3) Since I'm not sure about the chorus - but am sure about delay. I found delay in the "Variation" effects section. I want to find where that may be already adjusted in this Performance, so I look at the Common/Audio Mod/Control section and start turning assignable knobs with "Auto Select" turned ON. I could also look at the "All" summary as an alternative. What I find is that no Variation parameters are adjusted with superknob/assignable knobs. That's fine.
4) I look at PART1 again, look at its Mod/Control with "Auto Select" turned ON and start moving controllers. Again, I could use the "All" summary to look for "Var Send" - that's what I'm looking for. Since I have a Montage, I found the Ribbon controller is what is controlling the delay (var send). On MODX you'd have to either manually select "Ribbon" as the source - or change the source to "All" and scroll through the destinations since MODX has no ribbon controller.

Breakdown:

Select PART 1, choose "Edit", navigate to "Mod/Control" -> "Control Assign" screen, select "Display Filter" and change to "All". Then look at the "Destination X" (where X=1-16) list for the intended destination parameter. In this case, "Var Send". It's not on the first page. Press the next page arrow until you find the parameter you're after. Then touch this parameter to bring up and edit it. Touching the destination will bring a popup menu up to change the destination - just dismiss (exit) this. From here, you'll see the "Source" is shown as "Ribbon". Touch this "Source" and select "AsgnSw 1" to change from ribbon to A.SW1. Although this works as-is, I would change the curve from bipolar to unipolar then adjust the ratio (positive ratio) so that A.SW1 ON has the amount of delay I desire.
5) The last thing is to fix the button (A.SW1) so it is latched instead of momentary. Back to [PERFORMANCE] (HOME), then touch the Performance Name, then "Edit" so I am in the "Common/Audio" Edit area. "General" tab and change both Assign 1 and 2 switches to "Latch" mode.

If you want to do the same thing for "G Chorus" - you'll be dealing with the Dry/Wet parameter of that insertion effect. It is currently modulated by PART 1's Mod Wheel as the Source.

When in common edit mode for the Hybrid EP performance, I see that Tempo Cross Delay is designated as the effect. I actually don't hear any delay effect, but what I would prefer to do is change it to a phaser effect (that's the easy part) and have the assign 1 button turn it on and off. (can't figure that part out.) Can you help? Again, since I don't hear the delay effect, we might not hear the phaser effect either, so looking to correct that too. Thanks.

Here is a method to figure out what effects are assigned and when they can be heard.

There are System Effects (Reverb, Variation) - which apply to the entire Performance even when it contains all 16 Parts and the A/D Input Part.
There is the Master Effect - which also applies to the entire Performance when it contains all 16 Parts and the A/D Input Part

There are the Dual Insertion Effects which on the MODX you can apply to any of the first 8 Parts and to any four of the next 8 Parts (9-16)

The System Effect
From the HOME screen:
Press [EDIT]
Touch "Effect" > "Routing"
Here you will see the upper COMMON/AUDIO level of the architecture.
You see the VARIATION "Tempo Cross Delay" and the REVERB "Rev-X Hall"

The "Master FX" is OFF
The "Master EQ" is Flat

Part 1 Insertion Effects
If you touch the blue "Common" in the lower left corner, you'll see a Pop-up menu, Select "PART 1-Common"
Touch "Effect" > "Routing"

Here are the Insertion A and B assigned to the Hybrid EP.

How the Effects are being controlled
The "Tempo Cross Delay" is actually programmed to the Ribbon Controller (You may have noticed that your MODX does not have a Ribbon Controller, one is found on the MONTAGE and the Motif XF, where this sound originated, however).
On the MONTAGE/Motif XF touching the Ribbon Controller on the right side adds Tempo Delays that ping-pong between the right and left channels. Obviously, setup for a momentary effect - a big chord at the end of a run could be accentuated with a multiple Tempo repeat...

If you wish to change the VARIATION from the "Tempo Cross Delay" to a Phaser (as you say that is the easy part)... do so... Pick whichever Phaser you like...
We will show you how to turn the Assign Switch into a Ribbon Controller.

If you select PART 1 > Touch "Mod/Control" > "Control Assign"
The “Display Filter” will let you recall each Controller, as you do the Control Set (Source/Destination) will appear on the screen.
If you set the "Display Filter" = Ribbon Controller you will see that both the Variation Send (Destination 5) and Reverb Send (Destination 6) are set to be changed with the Ribbon Controller... Source is the Controller assigned to make the parameter change.

Here you can see that the Ribbon is responsible to increase the Send to the Variation Effect as you move left to right.
You can make the change here:
__ Change the SOURCE from "Ribbon" to 'Assign Sw1" shown below:

To make the Assign Switch a LATCH
Navigate to the COMMON > "General" screen
You do so by touching the blue boxes in the lower left corner and selecting "Common" from the Pop-up menu

Change the "Assign 1 Mode" from Momentary to Latch (green)

Now when you engage the Assign Sw 1 you will get your Phaser

Thank you, Mister Mister. Exactly what I was looking for!