This is part of it. Deflation means my money is worth more tomorrow than today. If due to 10% return just by keeping my money under my mattress due to deflation, why would I ever risk loaning it out into an investment opportunity. Essentially the risk-free return is now 10%, so for any investment to be worth wild it must return even higher than that. That's hard to do. The higher the deflation, the higher the baseline return needed. As a result any business that might've returned 5% is now no longer a viable option. So it reduces overall investment because it raises the bar for success.
Similar is for purchases. Why buy something now, when I can wait 6 months and get it cheaper. As a result everyone stops spending, and everyone stops investing. Now that there is no spending, there is no money to pay salaries, so people have less money to go around and so again money becomes more valuable and you get a downward cycle.
Imagine I said, at midnight january 1st any money you have in cash or in a bank will be worth 50% less. You'd buy up anything you could or invest in anything you could to get rid of cash and put it in hard currency. Even if someone had a business that would lose 10% over the year, that'd still be better than losing 50%.
Deflation is the opposite. Any cash or bank accounts you have will double. You'd sell your items, but nobody would want to buy. They could buy two couches with that money the next year.
Overall if money always is slowly decreasing in value, people want to find uses for it either consumption or investment which move the economy. If it's increasing in value (ie deflation) they wan to do the opposite. Hoard it.