Technical fashion designers take a lead designer's creations from the prototype stage through to full production. They work up specifications for new clothing and accessories, have samples made, and work with marketing and production facilities to ensure that finished pieces are made correctly and profitably.
Technical designers need to uphold the quality of their company's brands, not just by making sure that garments are solidly and consistently constructed, but by maintaining "fit continuity."
If they're responsible for a line of dress slacks (to choose an arbitrary example), all of the line's different slacks with the same measurements should fit customers the same way, and should fit customers very close to the same way as slacks with the same measurements that the company has released in recent years.
Technical design might be done "behind the scenes," but it's tremendously challenging and important.
Once the designers have figured out what shapes and cuts of material to use for an item, patternmakers create the master patterns for those shapes and cuts that guide further production. In the modern fashion industry, this is mostly done on the computer, but it still requires a steady hand and a sharp eye.