Normally when someone is acquitted by a jury, this will happen after they have been sent out at the end of a case having heard all of the evidence.
Sometimes however, at the end of the Prosecution case some or all of the charges get dismissed – for a high profile example see the Bill Roache case where Mr Roache was acquitted of one count at the end of the Prosecution case.
This is often called a ‘half time’ submission (as it occurs when all the Prosecution evidence has been given, but before the defence case, so half way through the trial), a submission of ‘no case to answer’ or, occasionally (amongst lawyers at least), a ‘Galbraith submission’ after the case of R v Galbraith  1 WLR 1039 which sets out the principles that are applied nowadays (although the principle that a case could be stopped at the end of the prosecution case pre-dates that by hundreds of years).
This will be heard with the jury out of the courtroom (as is the case with any legal argument – this is because if there is disputed evidence it would defeat the object of not admitting it if the jury hear it).
The basic idea is that due to problems with the evidence as it has come out there is not enough reliable evidence to go before a jury and so there is no need to hear from the defendant – the case should be stopped at that point.
What is the test?
The exact test is as follows:
“(1) If there is no evidence that the crime alleged has been committed by the defendant, there is no difficulty. The judge will of course stop the case. (2) The difficulty arises where there is some evidence but it is of a tenuous character, for example because of inherent weakness or vagueness or because it is inconsistent with other evidence. (a) Where the judge comes to the conclusion that the prosecution evidence, taken at its highest, is such that a jury properly directed could not properly convict upon it, it is his duty, upon a submission being made, to stop the case. (b) Where however the prosecution evidence is such that its strength or weakness depends on the view to be taken of a witness’s reliability or other matters which are generally speaking within the province of the jury and where on one possible view of the facts there is evidence upon which a jury could properly come to the conclusion that the defendant is guilty, then the judge should allow the matter to be tried by the jury…. There will of course, as always in this branch of the law, be borderline cases. They can safely be left to the discretion of the judge.”
If a Judge refuses the application, this does not mean that he thinks the defendant is guilty, just that it is possible that a jury could convict. In those circumstances, the fact that there was this argument will not be told to the jury. The only exception is if the Judge agrees that there is no case to answer on some of the counts of the indictment. In that case, the Judge will give a quick explanation and also say that this is not way means that the defendant is guilty on the remaining counts.
What’s the difference between an acquittal at half time and a ‘normal’ one?
The practical effect is nothing – it is still a not guilty verdict and neither is better or worse than the other. One major difference is that a Prosecutor can appeal a Judge’s finding that there is ‘no case to answer’ whereas they cannot appeal a jury’s verdict. This is going to be even rarer as the decision is often very fact specific and the Court of Appeal don’t generally like interfering with the factual findings of a Judge.
Can this happen the other way round?
No. However rubbish the defendant’s defence may be (or a Judge thinks it is) s/he cannot direct a jury to find someone guilty. A Judge can tell a jury that a particular defence cannot be relied upon, and so the inevitable consequence is that they are guilty, but a Judge can’t go that last step and stop a trial for that reason.