This is like a secret, many players still don’t know about this.
Engineering enchants like from the gloves and the back stack with other enchants, for example you can have Synapse Springs along with 50 Mastery and on back the Flexweave Underlay and the 70 spell penetration or 50 critical chance.
Usually for PVP the spell pen is better, for PVE the 50 crit chance is what you need.
This was different in Wrath of the Lich King since the back for example had more agility than the best normal agility enchant 22 > 23 agility for the engineering enchant. The glove enchant back then was using either the haste in PVE or the dmg shot in PVP.
Nitro Boots migrated from boots to the belt. There are no enchants for belts, except the “Ebonsteel Belt Buckle” that will create a prismatic slot for gems. This is made by blacksmiths and can be found in AH.
Any gem can be places here no matter the color.