To answer your question(s):
The penalty is assessed if the ineligible player is more than 1 yard down field on a forward pass play unless he is blocking a player that initiated within the 1 yard buffer zone.
If the defensive player breaks the contact with the ineligible player, the player must stop. He may not continue down field until the pass in completed.
Once a pass is complete all players may advance at their own discretion.
In the NFL there is no "screen pass exception", this is a NCAA rule where a player can be down field if the pass in completed behind the line of scrimmage.
NFL - Eligible Receiver
In order to catch a forward pass, a player must be an eligible receiver:
- They can line up in the backfield (ie running back, quarterback, or receiver)
- They can be on either end of the line of scrimmage (ie a tight end or split end)
Also in the NFL an eligible receiver cannot wear a number of 50-79 or 90-99 with more specific guidelines by position (Rule 5, Section 3, Article 1). These players have to declare to the referee they intend to be eligible on a play and the referee will announce that player as eligible. This is done so that the defense can figure out who can go out for a pass and needs to be accounted for. The player must line up as eligible if he reports as eligible.
NFL - Rule 8, Article 8
An in eligible player may not catch/or touch a pass, behind or in front of the line of scrimmage unless it is first touched by an eligible receiver or a defensive player.
NFL - Article 8, Section 3
This covers ineligible player downfield.
Article 1: Legal and Illegal Acts.
On a scrimmage play during which a legal forward pass is thrown, an ineligible offensive player, including a T-formation quarterback, is not permitted to move more than one yard beyond the line of scrimmage before the pass has been thrown.
Item 1: Legally Downfield.
An ineligible player is not illegally downfield if, after initiating contact with an opponent within one yard of the line of scrimmage during his initial charge:
(a) he moves more than one yard beyond the line while legally blocking or being blocked by an opponent; or
(b) after breaking legal contact with an opponent more than one yard beyond the line of scrimmage, he remains stationary until a forward pass is thrown; or
(c) after losing legal contact with an opponent more than one yard beyond the line of scrimmage, he is forced behind the line of scrimmage by an opponent, at which time he is again subject to normal blocking restrictions for an ineligible offensive player.
NCAA - Article 7-3-10-I and II
Ineligible Receiver Downfield ARTICLE 10.
No originally ineligible receiver shall be or have been more than three yards beyond the neutral zone until a legal forward pass that crosses the neutral zone has been thrown (A.R. 7-3-10-I and II).
This means that a pass thrown behind the neutral zone (line of scrimmage) does result in a penalty for ineligible players down field. Also the "buffer zone" in college is 3 yards vs. 1 yard in the NFL, but the NCAA does not have the blocking rule that allows a player to leave this buffer zone until the pass in complete.
2013 NFL Rule Book from NFL.com
NFL Rule on Offensive Lineman
2014 NCAA Football Rule Book
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