As previously stated, manipulating reverse thrust can bite you. The manufacturer doesn't want to share any liability for improper use so simply says to cancel reverse thrust for directional control problems. Aviation today is made as simple as possible so low time pilots can pass their check rides and get in the right seat as soon and as cheaply as possible.
Learning the finer points of piloting comes with experience and hopefully someone who will share with you what he has learned.
An olympic figure skating champion doesn't get there by reading the manual on how to use her skates, does she?