Unicorns- the 140+ private companies worldwide that are valued at $1 billion or more -- are well known for disrupting traditional industries, "hacking" old-school business models by using information, mobility and convenience to replace the need for physical (or human) assets. Facebook is the world's largest publisher, yet they publish no original content. Uber is the world's largest rental car company, yet they own no cars. Airbnb is the world's largest hospitality company, yet they own no hotels.

What is lesser known, but critical to their success, are the "behind the scenes" hacks that are making their business models possible. Specifically, their innovative approaches to strategic people initiatives -- hiring, onboarding, leadership training and knowledge transfer -- allow these disruptors to serve millions of customers with just thousands of employees. Some of these approaches seem magical in their simplicity, but then again, we are talking about Unicorns.

4 HR Hacks that Drive Unicorns' Success

Hack #1 Training at Scale: Uber relied on 1:1 driver training until a rapid expansion forced them to explore live group training sessions. Realizing quickly that even training 1,000 drivers at a time wouldn't meet their needs, they had to find a fix to get and keep drivers on the road. The hack? They built and now offer one-hour courses on demand, using quizzes to gauge effectiveness and surveys to collect driver feedback to continually improve the training. Today, Uber drivers complete over 30,000 courses per week on the mobile-friendly Mindflash LMS.

Hack #2 DIY Employee Handbooks: Slack is just over 2 years old, but the beloved collaboration software Unicorn has quickly grown from just a few founders to almost 250 employees. Similar to any organization in hypergrowth mode, they face the challenge of communicating benefits, explaining policies and of course, protecting their culture. The hack? Slack uses - you guessed it - Slack to create employee handbooks out of old conversation strings. In addition, they set up entire conversation channels for the sole purpose of helping new employees learn from the team's previous questions.

Hack #3 Being an Open Book: Airbnb's 33-year-old CEO, Brian Chesky, is a former art student who had no formal leadership training before founding the company. He now leads over 2,000 employees in 21 offices, serving 34,000 communities; and has done so through some intense PR and regulatory challenges. His leadership hack? Directly tapping insight from unexpected sources and sharing it. For example, he's sought out advice from a leading Sports Agent to learn the art of elite recruiting. He also makes learning a company-wide affair, taking these bits of insight and condensing them into a "Sunday Night Series," his weekly all-company email that summarizes what he's learned.

Hack # 4 Speed Dating: In her leadership roles at multiple Unicorns (Yammer, Mixpanel, Monster, Responsys), Misha McPherson has personally seen that, "With rapid growth, companies can quickly reach the stage of not knowing who new employees are, or even what different groups do." At Yammer and Mixpanel, Misha created cultural onboarding training to build empathy between teams. Every new hire class was taken through Speed Meets, where groups of 2-4 new hires were matched with "old timers" from across the company. While new hires rotated around to new groups every 15 minutes, the "old timers" had a chance to connect with each other over a year. This was a simple and inexpensive way to remove boundaries and onboard new hires into the entire company.

The common thread in these hacks? It's definitely not magic. Rather, it's rethinking old processes, accepting risk and leveraging technology to get things accomplished quickly without compromising on quality or impact. While these may not be the sexier hacks that generate headlines for these Unicorns, this is the kind of strategic thinking that secures higher valuations, faster than ever before, with dramatically fewer people.