Major League Baseball is making slight changes to its new pitch clock rules but will not alter the most significant portions of the mandates that have shaved 25 minutes off game times this spring, according to a memo obtained by ESPN.

The document -- the fifth of what a source called "clarification memos" sent by the league this spring -- was distributed Wednesday after players on the joint competition committee between the MLB Players Association and MLB requested various changes earlier in the week.

MLB, which has control over on-field rules, will continue with the parameters of the pitch clock that players have been using all spring: 15 seconds with the bases empty and 20 seconds with runners on base, plus the hitter needing to be "alert" in the batter's box with 8 seconds remaining.

"On one hand, we are prepared to make adjustments based on input," commissioner Rob Manfred said Tuesday. "On the other hand, we want to give it a chance to see how it plays out exactly over a period of adjustment in some regular-season games before we make any significant alterations."

Pitch clock violations have dipped precipitously over the first month of spring training, according to the memo, dropping from 2.03 per game in the first week to 1.03 this week, in line with the reduction in minor league testing last season. The average spring training game time has fallen from 3 hours, 1 minute to 2 hours, 36 minutes, the memo said.

The clarification memos have addressed more obscure issues and potential for attempts to circumvent the rules. The most important piece of the memo distributed Wednesday was the league changing replay review rules on potential violations of the infield shift ban. With the possibility of teams regularly issuing challenges after outs in hopes that one of the four infielders was positioned with his feet on the outfield grass -- which would negate the out and return the batter to the plate -- the memo said on batted balls that only the positioning of the defender fielding them could be challenged.

Other issues addressed include:

• On malfunctions of the PitchCom units that allow the pitcher and catcher to communicate electronically, players must immediately inform umpires, who can grant time and stop the ticking clock. PitchCom has become a vital tool for players since its introduction last year. Perhaps as soon as this week, sources said, the league is expected to approve its use by pitchers, who with it could call their own games.

• New standards will be enforced for bat boys and bat girls, whose ability to quickly retrieve equipment will help efforts to speed up the game, according to the memo. The league will evaluate the performances of bat boys and bat girls and could ask teams to replace them if their performance is considered substandard.

• On brushback pitches and "big swings" -- which either knock equipment out of place or land a player splayed out on the ground -- umpires will delay the start of the clock and, if the clock operator starts it early, have the ability to wave off the timer.

• In situations where pitchers find themselves away from the mound -- whether to cover first base or back up throws to home or third base in foul territory -- the 30-second between-batters clock will be delayed. It restarts when the pitcher making a play at first is back on the infield grass and one backing plays up is in fair territory.

• Leniency for catchers who end an inning on base or at-bat. Umpires could turn off the 2-minute, 30-second between-innings clock at the 30-second mark if the catcher has made a "reasonable effort" to abide by the timer. If it reaches that point, a catcher will be allowed to receive one warmup pitch from the pitcher and make a throw down to second base to ensure he, too, has warmed up his arm.

• Placing the onus on hitters to restart the clock if they take a timeout. Hitters may call time once in an at-bat, and previously the clock was starting from 15 or 20 when players stepped into the batter's box and were alert, leading to pitchers potentially holding the ball for long periods of time. Under the new guidelines, a player, regardless of where he is standing, must indicate to an umpire that he is ready to resume play, at which point the umpire will tell the operator to wind the clock.