This starts with:
value of setting "_BESClient_Download_LimitBytesPerSecond" of client
Which gets the value of that client setting.
value of setting "_BESClient_Download_LimitBytesPerSecond" of client | "-1"
Adds | "-1" which provides error handling. Essentially it means if whatever to the left produces an error, use the value to the right (in this case -1).
The reason why this is useful is because value of <setting> will produce an error if the setting does not exist.
Finally:
value of setting "_BESClient_Download_LimitBytesPerSecond" of client | "-1" != "0"
Returns true if the left hand side of != does not equal the right hand side.
It appears to be equivalent to the first version.