Tens of thousands of German workers downed tools again on Thursday as the country's largest union stepped up its battle for the right to a 28-hour week.

With more strikes on the horizon, here's what you need to know about IG Metall's groundbreaking campaign to rethink the work-life balance -- and what it could mean for the rest of Germany.

- What do they want? -

IG Metall represents some 3.9 million workers in Germany's crucial metal and electrical engineering industries.

As in past industry-wide negotiations with employers, the union is pushing for a wage hike -- this time seeking a whopping six-percent increase.

But what stands out in the current showdown has been the emphasis on giving employees freedom to tailor their work hours to their personal lives.

The union wants all workers to have the option of switching from a 35- to a 28-hour week for a two-year period, with a guaranteed return to full-time work afterwards.

In certain cases -- and this has been the most headline-grabbing demand -- the union says employers should make up some of the salary loss that would result from clocking up fewer hours.

It wants those caring for young children or elderly relatives, for example, to receive an extra 200 euros (around $240) a month.

And shift workers or others whose working hours can weigh on health should be entitled to an additional 750 euros annually, it adds.

- What are the arguments for a shorter week? -

IG Metall says flexible working time has so far mainly benefited employers who got staff to put in longer days.

But with Europe's top economy humming and unemployment at a record-low, it believes the time is right for a radical shake-up.

"Workers aren't only workers, they have personal lives, children, old parents," Berlin IG Metall chief Olivier Hoebel told strikers at a demonstration on Monday. "Working life can't only be about sacrifice."

IG Metall believes its proposals would especially benefit women, large numbers of whom work part-time for family reasons and currently don't have an automatic way back to full-time employment when their situation changes.

- How have employers reacted? -

With a firm 'no'.

The Gesamtmetall employers' federation has predictably balked at the suggestion of paying staff extra to work less.

It has dismissed the proposals as "too costly" and "unfair" to those already in part-time work under less generous conditions.

It says introducing the compensation measure would be discriminatory and open companies up to legal action.

After two rounds of negotiations, employers have so far offered a two-percent wage increase, but no progress has been made on the 28-hour issue.

- What would be the impact? -

Where IG Metall goes, others tend to follow.

Europe's largest trade union was instrumental in pushing through a 35-hour work week in the 1990s, and employers across Germany are closely watching to see if the next labour revolution is around the corner.

Already the call for a shorter week has triggered heated debate about quality of life and the future of work in a world where jobs are increasingly automated.

Supporters have praised the union's proposals as "very modern" and said they could help companies hang on to their best and brightest.

But critics have countered that a reduced week could exacerbate a shortage of skilled workers, while smaller firms in particular might struggle to meet production targets.

"If it would be replicated throughout the economy, it could do serious damage," said Holger Schmieding, chief economist at Berenberg bank​.

Gesamtmetall estimates some 1.5 million workers would be eligible for the proposed compensation if they chose the 28-hour route. IG Metall however believes the actual take-up would be far lower.

- Where to go from here? -

Tensions were running high as a third round of talks began Thursday.

Mobilised by IG Metall, more than 250,000 workers took part in a series of hours-long "warning strikes" this week at hundreds of companies, including BMW, Daimler and Siemens, and more shutdowns were planned for Friday.

The union has vowed to go further by calling for day-long walkouts if the standoff continues, even threatening to stage its first nationwide, open-ended strike since 2003.

"If employers won't drop their veto stance and start talking to us about working time, we will have to bring out the big guns," said IG Metall chief Joerg Hofmann.