The overall theme of this spec is to make the engineer able to sustain themselves and keep targets on them.
Dashing is built into the spec and keeping volatility between 30-70 will be optimal.

I see alot of builds with multiple damage abilities for tanking. I see no need for this. Since adding more damaging abilities only adds to complexity and reducing the amount of active Damage mitigation. The tanks main job is to survive and to control the mobs.
is the main volatility generator and will allow you to consistently keep yourself within the optimal 30-70 volatility margin. Going to tier 4 makes creating volatility easier.
Main volatility dump. As a tank i tend to prefer large aoe so catching an add will be easier
his taunt will sometimes take presure off of you and the reason to going to tier 4 is for the extra deflect chance. The bot ability should be used constantly to make sure that your damage is mitgated.
is here to add yet another ability to keep your shield up and with the tier 4 increases your shield even more. Using tier 8 gives you bonus to shields and also gives you access to a juicy mitgation on top of that.
this is the first of many "oh shit" buttons that you have in this spec. Adds all over the place hit boss about to hit dps. Going to tier 4 is more of a preference than a must have.
Another "Oh Shit" button in case you get hit by a cc cupcake.
This is a hard CC meant to help with interrupt armor and CC any adds
Use on Cooldown this will allow you to take reduced damage tier 4 is necessary.

Utility
Dashing will help with keeping your shield up and volatility up. upgrading your Dashing will also promote solid game play via dodging telegraphs.
Repair bot is their to increase shields and keep you buffed up
Hybrid S/U
Shield increase capacity is a no brainer considering you can constantly heal your own shield.
Dirty Tricks will help reduce more damage while helpin hand will increase healing when you need it.
Survival instincts is in because it allows you to regain health while deflecting again adding to your overall surviveabilty.
Support
Deflect with this spec allows you to heal yourself
Enmity is thier to help maintain threat while Rejuvenating Rain is meant to sustain you
Try and hurt me with boosted armor makes you even harder to kill
The last 2 points are personal preference so I like the healing of life steal but they are honestly replaceable with whatever you see fit.

Overall this spec is meant to keep the threat on you while being an unstoppable yet agile juggernaut.