This excerpt (source below) is the best defence I have found so far for the theory of the Initial Physical Evidence of Speaking in Tongues. I put this excerpt online, not to propagate this doctrine, but rather for academic purposes.
I myself have prayed for people to be filled by the Holy Spirit, who started shaking under the power of the Holy Spirit but they did not always start to speak in tongues (immediately). However, this does not mean that the theory of the Initial Physical Evidence is wrong, since it is possible that they spoke/will speak in tongues afterwards.
Whether you believe in the Initial Physical Evidence, or not, is totally up to you.
‘PROPHECY THE EVIDENCE BEFORE PENTECOST
Before the day of Pentecost, the people of God constituted a nation. The door was open to others to join the nation, but anyone seeking salvation would have to become circumcised and thus become a Jew. In the words of Christ, “salvation is of the Jews.” John 4:22. Of course this nation had its language, and one language was sufficient then in which to converse with God’s people. Thus it happened that when men received the mighty anointing from on high, that experience was evidenced by prophecy. Prophecy is speaking one’s own language in the power of the Spirit.
INSTANCES OF PROPHECY
When seventy elders received of the spirit of Moses they prophesied. Numbers 11:25. When the Holy Spirit came upon Saul, the first king of Israel, he prophesied. 1 Samuel 10:10. Hence the proverb, “Is Saul also among the prophets?” The prophets among whom he was, themselves prophesied. 1 Samuel 10:5; 19:20. When the Holy Spirit came upon prophets of old, they “spake as they were moved by the Holy Ghost.” 2 Peter 1:21. Joel prophesied that the time would come when God would pour out His Spirit upon all flesh, and they would prophesy. Joel 2:28-29. When Elisabeth and Zacharias, the parents of John the Baptist, were filled with the Holy Spirit they, too, prophesied wonderful words in the Spirit. Luke 1:41, 42, 67. Thus we see that prophecy was the evidence or sign of the reception of the Holy Spirit’s power before the day of Pentecost.
TONGUES THE EVIDENCE AT PENTECOST
Now at the beginning of the church dispensation, the field of operation for the gospel ministry was widened to include all nations. Jesus had said, “Go into all the world and preach the Gospel.” “Go ye therefore and teach all nations.” The power of the holy Ghost was to enable them to witness both at Jerusalem, Judea, Samaria, and the uttermost parts of the earth. The disciples were instructed to wait until they received power from on high before beginning this world-wide ministry. It was very fitting then that instead of the mother tongue in prophecy being used as an evidence of the reception of the Holy Ghost, in the church dispensation there should come a divine power which could enable them to speak in other tongues, many and carried. On the day of Pentecost there were about fifteen different nationalities present. Among the 120 disciples who were all filled with the Holy Ghost and spoke in other tongues, all fifteen languages were spoken and understood by these nationals who were present. “How hear we every man in our own tongue wherein we were born?” Acts 2:8.
TONGUES ARE CONTINUATION OF PROPHECY AS EVIDENCE
This speaking in other tongues then became the sign and evidence that the Holy Spirit had descended upon New Testament Christians. As a matter of fact, Peter considered that this speaking in other tongues was in reality prophesying. He said, “This is that which was spoken by the prophet Joel. And it shall come to pass in the last days, saith God, I will pour out of my Spirit upon all flesh: and your sons and your daughters shall prophesy, and your young men shall see visions, and your old men shall dream dreams, and on my servants and on my handmaidens I will pour out in those days of my Spirit; and they shall prophesy.” Acts 2:16-18. In reality, we are told in 1 Cor. 14:5 that tongues and interpretation are the equivalent of prophecy. As prophecy was the evidence of baptism when the church or people of God was national, so tongues is the evidence while the church is international.
THE EVIDENCE AT CORNELIUS’ HOUSEHOLD
Confirmation of this conclusion is found in the experience of the Gentiles that had gathered together at Cornelius’ household. “While Peter yet spake unto them the Holy Ghost fell on all them that heard the Word, and they of the circumcision which believed were astonished, as many as came with Peter, because that on the Gentiles also was poured out the gift of the Holy Ghost; for they heard them speak with tongues and magnify God.” Acts 10:44-46. This was the introduction of this Holy Spirit experience to the Gentiles, and its introduction was evidenced in the very same way as was its coming upon the Jews on the day of Pentecost.
THE EVIDENCE IN THE EXPERIENCE OF PAUL, THE EPHESIANS AND THE CORINTHIANS
It cannot be said that this evidence of tongues was manifest only at the initial outpouring of the Spirit upon the Jews and the Gentiles. Paul was a Jew and was filled with the Holy Spirit and said “I thank my God I speak with tongues more than ye all.” 1 Cor. 14:18. The Gentiles at Ephesus likewise received the Holy Spirit and “spake with tongues and prophesied.” Acts 19:6. The Gentiles at Corinth also received this same experience with speaking in other tongues, for the 12th, 13th and 14th chapters of First Corinthians reveal this fact.
TONGUES THE EVIDENCE TODAY
It is thus manifest that the initial physical evidence of the baptism of the Holy Spirit is the speaking in other tongues as the Spirit gives utterance. On the day of Pentecost “they were all filled with the Holy Ghost and began to speak with other tongues as the Spirit gave utterance.” At Cornelius’ household, “the Holy Ghost fell on all them that heard the word; for they heard them speak with tongues.” At Ephesus, the grammatical inference is that all twelve men who received the Spirit spoke with tongues and prophesied. Therefore, all who receive the Baptism in the Spirit today also speak with tongues.’
Source: Ralph M. Riggs; The Spirit Himself (1949); Chapter 11: The Baptism in the Holy Spirit, the Initial Physical Evidence; p. 85-89.