Let me toss my description into the ring as well.
Driving into another robot: Generally not illegal unless it results in you flipping the opponent, or performing some other illegal act.
Verdict: Ramming is a staple of defense.
Pushing: Generally not illegal to push. If you use concave elements or otherwise meet the definition of “grapple” is illegal.
Verdict: If you can push and it is advantageous to do so, by all means push away.
Special note: Ramps and wedges are often used to make pushing easier. Think battlebots. So long as you don’t flip them, it is legal.
Trapping and Pinning: These are the same thing. The moment you trap an opponent in a space of one tile or less, a pinning count starts. Pinning is on a per robot basis, so you can move from pinning one robot for 4.5 seconds to pinning their teammate with impunity, so long as you don’t break any other rules.
Verdict: Trapping or Pinning is viable so long as you do so within the rules.
Special Note: Trapping is fully legal in the last 10 seconds.
Boxing Out: Defending a position is not illegal. Denying an opponent access to the roller is legal, so long as they can escape and leave. Defending is not trapping, unless they cannot escape and go elsewhere.
Verdict: You can Box Out with impunity and it is advisable to do so when it is helpful.
Reading what isn’t and what is in the rulebook takes a while to come across this understanding. However, watching play for a few seasons, understanding the rules, and taking the Referee Certification, will best prepare you for strategizing and driving in VEX.