Dropcatch software is a tool that allow you to send domain registration request to registrar directly via API interface. Normally, you would be required to refill your account first before using it. If the drop catch is success, the registration fee will be direct deducted from your account balance and the domain directly assigned into your account.
 
There are 3 levels of players in the game of drop catching.

Tier 1: Registrar Level (Those Registrars are not open for public, they existed for the sole purpose of drop-catching)
Tier 2: Registrar's reseller API level
Tier 3: Hand registration manually
 
Top level Registry imposed a concurrent connection limit to each second level registrar. Every registrar has to obey the connection limits. So if they want to increase their chance of success in drop catch. They need to increase the number of registrars they own, in fact thousands of private registrars just created solely for this drop catch purpose. The companies like dropcatch.com, snapnames.com are Tier 1 players in the game of drop catch.
 

Most of the drop catch software are fall into tier 2 and they will not be able to compete with Tier 1 players, especially for .com domains. Anyway, the dropcatch software user still have a chance to compete with Tier 2 players and Tier 3 players. So, if the domain the user come after is not pursued by tier 1 players, they may still have a chance to catch it.
 
 
More information regarding dropcatch software can be found in the links below.