A classic Black Hat ASO technique is buying downloads. 

The number of downloads is the first important metric the app store tracks, so an increase in downloads (via buying them) is a sure way of letting them know that something is happening to your app. 

However, they also check the number of uninstalls and engagements (reviews included) your app will receive to validate if your downloads are real. 

Take note that when you decide to buy downloads, you will need to buy a lot. 

These downloads should also come from the country where you are concentrating your marketing efforts. 

For example, you are a UK based app wanting to grow in the USA. You should buy downloads from the USA and not from New Zealand. 

Buying downloads though can be useful growth hacking tool. Once you achieve top ranking in NZ for example, you might get featured in another country as well since the app store editors all share their news. 

However, if you do want to try this technique, don’t forget that you should also buy or guarantee reviews so it looks like your downloads are organic.