To acquire prospects. Loads of them.
Draft position is one part.
More money to spend on the draft as a last place team (this one is super egregious and ripe for abuse.)
More money to spend on Latin American Academies as a last place team (Also super egregious and ripe for abuse)
Selling anyone over the age of 25 that showed a pulse for prospects. Doesn't matter how good the player is, one of those prospects might turn out to be a middle reliever or something.
Any prospects that did show promise were restricted to the Minor Leagues until the second half of last season.
Here is a better article on why the Astros did what they did:
http://grantland.com/features/hoarding-prospects-being-horrible-houston-astros/
Yes, tanking for a draft pick doesn't work. But tanking for prospects, and strip mining your team, can work when done right.
If you feel that tanking should only be reserved for a draft pick, whatever. Call it strip mining, call it something else, but they weren't going to suck during the second half of this decade, and the big reason why is because they sucked in the first half of this decade.
They purposely fielded a bad team, that's tanking. Why they are tanking is irrelevant.