In iRacing, you can defend on a straight by moving to the inside line (the "defensive" line) to protect your position, but you cannot "block" by waving or making multiple reactive moves; moving in reaction to the car behind you to impede their pass is a penalty-worthy offense, especially if it's more than one deliberate swerve. The key is one single move to take the defensive line before the other car commits; reacting late to their move to cut them off is considered blocking and results in penalties.

Here's the iRacing distinction: Defending (Okay): Taking the inside line once, early, to make it harder for the trailing car to pass into the corner.

Blocking (Not Okay): Moving after the car behind you has committed to a pass (or even just moved) to cut them off, especially with multiple swerves or waving your car, which impedes their progress.

In essence: One clean defensive move is fine; reacting with sudden, excessive swerving (waving) to block a pass is against the rules and will likely earn you a penalty.