The rule 'Internal Horizontal Port Scan NMAP Top 20' is designed to detect internal reconnaissance activities on a network by identifying instances where an internal host attempts to connect with 250 or more unique destination IP addresses using one of the top 20 ports associated with NMAP scans. Such behavior can indicate potential malicious scanning activities or misconfigurations within the network. The detection is made possible through analysis of AWS CloudWatch Logs for VPC flow, wherein the rule parses network traffic data, looking specifically for traffic patterns that meet the established criteria of frequency and port usage. The output includes a warning for security teams to analyze and respond to these potentially harmful activities.